The battle of Pliska was one of the worst defeats in Byzantine history. It deterred Byzantine rulers from sending their troops north of the Balkans for more than 150 years afterwards, which increased the influence and spread of the Bulgars/Bulgarians to the west and south of the Balkan Peninsula, resulting in a great territorial enlargement of the First Bulgarian Empire.
During the rule of khan Krum the centralization of the khan's power reached its peak. The Bulgars did not limit their wars only to Byzantium; they also waged wars in the west of the Balkan Peninsula, and those wars transformed from defensive to aggressive and invasive. During the first years of his rule, Krum had to attend to his north-west borders where at the beginning of the 9th century the political situation changed due to the expansion of the Frankish Empire in the Middle Danubian region and the repulsion of the weak remnants of the Avar Khaganate towards the east beyond the Tisa River after the decisive victory of Charlemagne over the Avars in 803. This last event presented an occasion for Krum to put an end to the Avar possessions.
Bulgar warriors. Scene from reenactment of the battle,
26 July 2006. Photo credit: Klearchos Kapoutsis
In 805, the Bulgars killed and captured the remaining Avars, and annexed their lands in today's Eastern Hungary and Transylvania to Bulgaria. The Bulgars put the kagan to flight and captured a host of Avar soldiers; years later, the latter would serve in the Bulgars' wars against Byzantium. The Slav tribes that lived in those lands, after being freed from the Avar rule, recognized the power of the Bulgar Khan.  Thus the Bulgar state became a neighbour of the Frankish Empire, with the recognized border starting from the estuary of Sava upstream on the Danube to the Tisa River, then upstream the whole length of the Tisa and along the Prut River to the North Besarabian trench at Leovo, along this trench which in the east reaches to the Dnester River near the town of Benderi to the south-east, finishing at the Black Sea coast. Of course, given those borders to the northwest, it is beyond doubt that the Bulgars had succeeded in annexing the lands along the Mlava and Morava Rivers to the border of the Servian tribes; the expansion of Bulgaria in this direction happened earlier together with the gradual subsiding of the Avar rule there in the 8th century. 
One of the main episodes in this struggle was the uprising of the Peloponnese Slavs in 805 (or 807) who plundered and devastated the neighbouring villages, occupied the outskirts of the town of Patri, and besieged the town, in alliance with the Arabs. However, the siege was unsuccessful and the Slavs were defeated. The Byzantines thought that their victory was entirely due to the blessing of the Apostle Saint Andreas, the patron of the town of Patri. When Nicephorus learned about this, he decided that, because the victory was achieved thanks to St. Andreas, all the trophies, taken from the Slavs belonged to him, the Emperor. After that, he ordered that all Slavs who besieged Patri, together with their families, kins, and possessions, be bound to the soil of the church St. Andreas in the Patri Mitropoly. From then on, the Slavs belonging to this mitropoly were obliged to pay the expenses of the strategos, archons, patricians, and all dignitaries, sent by the Emperor to the church land. The fate of the Peloponnese Slavs signaled to the other Slavs in the Empire, that a similar fate could be expected by them if they did not immediately receive help from the outside. Such help they could receive only from the Bulgars who were already a force to be reckoned with on the peninsula. On their side, the Bulgars did not miss an occasion to show their readiness to help, especially towards the Macedonian Slavs.
Such relations between Macedonian Slavs and Bulgars can be surmised from the expedition of Nicephorus against the Bulgars in 807. He only reached Adrianopolis (today Edirne), a Byzantine town close to the Constantinopolis, returned back to the capital, and canceled the campaign after learning of a conspiracy by the courtiers and military against him there. Theophanes  presents this expedition as senseless; however, the reason can easily be found in the relations between the Macedonian Slavs and Bulgars. That abortive attack, however, gave reason for the Bulgarian Khan Krum to undertake military operations against the Byzantine Empire. The main objective was an extension to the south and south-west. In the next year a Bulgarian army penetrated the Struma valley and defeated the Byzantines. The Bulgarian troops captured 1,100 litres (360 kg) of gold, earmarked for soldiers' pay, and killed many enemy soldiers including all strategos and most of the commanders because they were gathered to receive their pay.  It is scarcely possible that this surprising attack had been undertaken only for robbing gold; on the contrary, as with the similar attack of 789, one can see a systematic effort by the Bulgars to penetrate towards the Aegean Sea and detach the western regions of Byzantium. Therefore, the Bulgars wanted to weaken this military centre, which is supported by the fact the in the spring of the following year, Krum undertook a serious military expedition in the same direction. Just before Easter in 809 the Khan besieged the strong fortress of Serdica (today Sofia) and seized the city, killing the whole garrison of 6,000. [11, p. 342]
The Byzantine Catastrophy
The Pliska expedition
In 811, the Byzantine Emperor organised a large campaign to conquer Bulgaria once and for all. His preparations were long and careful; troops were collected from throughout the Empire. There was no danger from the Saracens at the moment; so he gathered an enormous army from the Anatolian and European themata with their strategi, and the imperial bodyguard (the tagmata). The troops of the Asiatic themes had been transported from beyond the Bosphorus; Romanus, general of the Anatolians, and Leo, general of the Armenians, were summoned to attack the Bulgars, as their presence was no longer required in Asia to repel the Saracens [11, p. 343]. They were joined by a number of irregular troops, armed with slings and clubs, who expected a swift victory and plunder. The conquest was supposed to be easy, and most of the high-ranking officials and aristocrats accompanied Nicephorus, including his son Stauracius and his brother-in-law Michael I Rangabe, all patricians, commanders, officials, all divisions, and commanders' sons who were above 15 years of age of which last he composed a division of his son, and called them Worthies (Hikanatoi). [2, p.148] The whole Byzantine army is estimated to have been up to 60,000  or 80,000  soldiers.
Byzantine camp. Scene from reenactment of the battle, 26 July 2006. Photo credit: Klearchos Kapoutsis
In May 811 , the great expedition left Constantinople, led by the Emperor himself and his son, Stauracius, and set up camp at the fortress of Marcelae (present-day Karnobat) near the Bulgarian frontier where it stopped to gather the various detachments coming from the different parts of the Empire. The period of stay at Marcelae is not known: estimates range from several days to several weeks. Judging from the fact that the Byzantine Empire was very large and time was needed especially for troops from Asia (e.g. the Armenians), it is safer to take the higher estimate, which supposes that the stay at Marcelae took the better part of June and/or early July. This is confirmed by the events that happened at Marcelae. After learning that such a large army was gathering at his border, Krum assessed the situation, estimated that he could not repulse the enemy, and sent ambassadors to Marcelae begging humbly for peace which Nicephorus haughtily rejected; he was distrustful of Bulgar promises and confident of victory. [9, p. 56] Theophanes disapprovingly writes that the Emperor was deterred by his own "ill thoughts" and the suggestions of those of his advisors who were thinking like him.[1, p. 486] Some of his military chiefs considered the invasion of Bulgaria to be imprudent and too risky but Nicephorus was convinced of his ultimate success, counting mainly on the luck and wisdom of his son Stauracius. At this time, a courtier close to Nicephorus, by the name of Byzantios, escaped from Marcelae for unknown reasons and went to Krum, taking with him the Imperial apparel and 100 litres (about 33 kg) of gold; many considered this as a bad omen for Nicephorus.
Another bad omen was the unfavorable period of the year, coinciding with the heliacal rising of Sirius, the Dog Star.  "It was the devastating rising of the Dog" [1, p. 486], the Dog Days, considered to be an evil time "when the seas boiled, wine turned sour, dogs grew mad, and all creatures became languid, causing to man burning fevers, hysterics, and phrensies".  To Greeks this signified certain emanations through which the Dog Star exerted its malign influence. People suffering its effects were said to be 'star-struck' (astroboletos).  The Dog Star caused a "reckless bravery of the impertinent coward [Nicephorus]" and made him behave like a madman, frequently shouting challenges and then realizing that some supernatural power, "God or his enemy" (the devil), pulled him against his will. [1, p. 486]
The sky above Pliska at dawn, 03:06 a.m. on July 23, 811. 
The march towards the Bulgarian capital Pliska is not well described. Traditional historical treatments follow Theophanes who records that Byzantines penetrated Bulgarian territory on July 20 , [8, p. 331], .
At the time of the battle, the Bulgarian border was situated to the south of the Balkan Mountains, and Krum controlled important towns and garrisons on the southern side, including some that were very close to Marcelae. It is probable that by "Bulgarian territory" Theophanes means the lands north of the Balkan, since it is hard to imagine that a Byzantine historian would acknowledge a barbaric tribe owning land that has always been considered part of Byzantium. During the first millennium, the territory of northern Bulgaria (Moesia) was covered with an unbroken forest, known in Europe as Magna Silva Bulgarica. The forest was especially dense and impassable in the discussed region: Veregava and the plains and valleys at its foothills. It further slowed the march: the large army moved in columns along the narrow forest paths, the cavalry frequently dismounting at the steep slopes. Because this was a hostile territory, light cavalry scouts were sent ahead to spy out the army's line of march, the position of enemy forces and fortifications, the availability of wood and water, fodder and food, and were responsible for providing the commanders of the Byzantine forces with sufficient information for them to plan their route and the marching camps.
Above: Emperor Nicephorus enters
Bulgaria with his army.
Below: The captured Nicephorus
is presented to Krum.
Miniatures from the
The crossing, difficult for such a multitudinous army, would inevitably occupy some time. Approximate distances and timing are listed in the following table.
|Rish Pass||Vărbitsa Pass||Kotel Pass||Luda Kamchia|
|Total distance  (km)||91.66||117.18||145.12||98.75|
|Distance in pass (km)||12.91||25.85||25.01||0|
About distances, the following must be borne in mind: while distances in the passes are relatively accurate because they were measured by following the contour of the pass, total distances are underestimated by 10-30 km because level terrain was measured on a straight line, since it is impossible to guess the exact route on level ground. For the timings, one must consider a march of 25 km to be both long and tiring for men and horses, and although this rate could have been maintained as an average in some cases, terrain, weather and the quality of the roads, tracks or paths used by the army will all have played a role, so that very considerable variations must have been usual. The average length of a day's march for infantry or combined forces was probably rarely more than 19-23 km, which has been an average for most infantry forces throughout recorded history; and this figure would more often than not be reduced if very large numbers, which had to be kept together, were involved. From the above mentioned, and taking into account that the Byzantine army was very large, one can take the lower estimate (18 km per day) as the rate of march, reducing it further to 11 km per day for march in a pass. Timings in the table are calculated on the above assumption; as seen, the march from Marcelae to Pliska could have taken 5.5 to 9 days. This defines the period of departing from Marcelae as July 11 to July 14, according to Theophanes , or July 2 to July 5, according to Scriptor Incertus .
Nicephorus intended to confuse the Bulgars, and over the next ten days launched several feigned attacks, which were immediately called back. The Byzantines met little resistance [4, vol. 3, sheet 1, p.17] and in three days they reached the capital, where they met a 12,000 army of elite soldiers who guarded the stronghold. The Bulgars were defeated and most of them perished. Another hastily assembled army of 50,000 soldiers had a similar fate. [2, p.148-149] On 23 July the Byzantines quickly captured the defenseless capital. The city was sacked and the countryside destroyed. [6, p.372-373]  Khan Krum attempted once more to negotiate for peace:
Here you are, you have won. So take what you please and go with peace. [1, p.487]
Nicephorus, overconfident after his success, ignored him. He believed that Bulgaria was thoroughly defeated and conquered.
Byzantines attack the Bulgar stronghold. Scene from reenactment of the battle, 26 July 2006. Photo credit: Klearchos Kapoutsis
Michael the Syrian, patriarch of the Syrians Jacobites in XIIth century described in his Chronicle the brutalities and atrocities of the Byzantine Emperor: “Nicephorus, emperor of the Romans, walked in Bulgars land: he was victorious and killed a great number of them. He reached their capital, took it over and devastated it. His savagery went to such a point that he ordered to bring their small children, got them tied down on earth and made thresh grain stones to smash them.” [4, vol. 3, sheet 1, p.17] The Byzantine soldiers looted and plundered; burnt down the unharvested fields, cut the sinews of the oxen, slaughtered sheep and pigs. [2, p.150] The Emperor took over Krum's treasury, locked it and did not allow his troops to reach it at the same time cutting noses and other appendages of soldiers who touched the trophies. . At the end, Nicephorus ordered his troops to burn down Krum's residence. [1, p.490] [6, p. 372-373]
The battle in the pass
Nicehorus is reported to have said: [1, p. 490-491]
Even if we had wings we could not have escaped from peril.
It must be noted that nights in this period were dark and moonless, with the moon late in the fourth or early in the first quarter, having entered the -13,746 lunation on July 24, 07:17 local time.  For several nights, in which they could not see even the shadows of the Bulgars that were following and surrounding them, a noise of troop movements and clang of arms kept Nicephorus and his companions in a feverish restlessness and brought them to an utter exhaustion. [1, p. 490-491] On July 26 [1, p. 490-491], Saturday [1, p. 490-491] [2, p. 152], the Bulgars gathered their troops and tightened the noose around the trapped enemy. At dawn, they rushed down and started to kill the panicked and totally confused Byzantine s, who fruitlessly resisted for a short time before perishing. Upon seeing their comrades' fate, the next units immediately ran away.
Above: The war of Knyaz Krum. Below: The army of Khan Krum chases and wounds Nikephorus's son and heir Staurakius.
In their retreat, the Byzantine forces hit a swampy river which was difficult to cross. As they could not find a ford quickly enough, many Byzantines fell into the river. The first ones stalled in the mud with their horses and were trampled by those who came next. The river was filled with so many dead men and horses that the chasing Bulgars easily passed over them and continued the pursuit. Those who passed through the river reached a wooden wall which was high and thick. The Byzantines left their horses and began climbing the wall with hands and legs and hung over the other side. The Bulgars had dug a deep moat from the outer side and when the Byzantine soldiers were getting across the ramparts, they fell from the high wall, breaking their limbs. Some of them died instantly, others hobbled some time before falling to the ground and dying from thirst and hunger. The Byzantine troops burnt the wall at several places but as they were rushing to get across it, they too fell into the moat along with the burning parts of the palisade. The anonymous narrator laments on this event, in which, it seems, most of the Worthies (the youngest soldiers) were killed:
Who will not weep when he hears this? Who will not cry? Thus perished the commanders' sons both of the old and of the young ones who were a whole multitude, in the blossom of their youth, and they had beautiful bodies that shined with whiteness, with golden hairs and beards, with handsome faces. Some of them had just been engaged to women, distinguished with nobility and beauty. All perished there: some brought down by sword, others drowned in the river, third fell from the rampart, and still others burned in the moat. Only a few of them escaped but even they, after they arrived in their homes, almost all of them died.
—Scriptor Incertus, p. 148-149
Among those killed were the patricians Aecius, Peter, Sisnius, Tryphillios, Theodosius Salivaras (the patrician Eparchos [Prefect] of the capital), Romanus (the patrician and strategos of the theme Anatolic), and many protospatharios, spatharios, and archons of the tagmata, the domesticos of the Excubitors, the droungarios of the Imperial Watch, the strategos of the Thracian army, archons of themes together with innumerable soldiers. All arms and Imperial treasures were lost. [1, p. 492] Nicephorus' son Stauracius was carried to safety by the imperial bodyguard after receiving a paralyzing wound to his neck. [1, p. 489-492] [6, p. 373]. Only a few survived the defeat, one of them being Nicephorus' brother-in-law Michael Rangabe; the majority of those who survived died shortly after they arrived at their homes.
The most notable person to be killed, however, was Emperor Nicephorus. According to Christian historians, the Byzantine soldiers hated him so much that they killed him in some way or another: some say that the Christians (Byzantines) killed him with stones after he fell down while the eunuchs in his entourage (parakoimomenous) died either in the fire of the burning ramparts or were killed with swords [1, p. 491]; either the Byzantines killed him themselves or, when the barbarians started to kill him, the Byzantines finished the killing of the torturer [6, p. 373]; in any case, Nicephorus was killed by a Roman [Byzantine]. [4, vol. III, p. 373] However, old Bulgarian sources say explicitly and unequivocally that Nicephorus was killed by the Bulgars, even by Krum himself. Thus, in the old-Bulgarian translation of the Mannases Chronicle, writing in general about the Nicephorus catastrophe in 811, one reads:
This tsar Nicephorus came into the Bulgar land during Kniaz Krum['s reign] and at first he apparently vanquished him, and plundered the estate bearing his [Krum's] name. After this, Krum gathered those who were left after the defeat, and he attacked the tsar during the night, and not only defeated the Greeks, but he [Krum] himself cut the head of the tsar, and he cased his head in silver, and poured wine in it, and he gave it to the Bulgars to drink from it. [3, p. 143]
Further in this chronicle, under two miniatures, illustrating the above text, it is written that "Kniaz Krum" caught tsar Nicephorus and cut his head. [3, p. 145] In the Arabian Synaxarium (Prologue), that had copied the description of the said battle almost literally from the Greek Synaxarium, under the month of Tammuz (July) day 23, there is the following synopsis:
In this day, we mention our Christian brothers, who died in the Bulgar lands in the days of tsar Nicephorus who set out with his Army during the ninth year of his reign against the Bulgars, attacked them suddenly, and was deigned with victory at first, and [Nicephorus] won a great victory. But what came to pass after this, is not to be muted but deserves cry and lament. It happened so that, one night, the Bulgars taking advantage of the carelessness of the Greeks, attacked their army, killed the tsar and many other commanders. Those who received deadly blows transcended immediately from our world; those for whom the blows were not deadly hid in the wooded and overgrown places; those who were captured alive suffered numerous tortures because they refused to deny Our Lord Jesus Christ; for some of them their heads were cut with sword; others were deprived of their present life with strangling; thirds were wounded with numerous arrows and transcended from this life. As for the rest, they were imprisoned in dungeons and sentenced to hunger and thirst. In this way, they freed themselves from this world and were wreathed with martyrs' wreaths. 
Khan Krum receives the head of the Byzantine Emperor Nikephorus. Painting by Nikolay Pavlovich (1835-1894)
According to tradition, Krum had the Emperor's skull lined with silver and used it as a drinking cup. From the Byzantine (Christian) point of view, this act is an expression of the barbaric Bulgar customs, and is nothing more than sacrilege and a humiliation of Nicephorus. One must take into account, however, that according to the pagan religion of the Bulgars, the strength of the enemy, residing in his head, dissolves in the wine, and transfers to the blood of the person who drinks from the skull, making him invincible. The most powerful ruler of Europe had been vanquished, and Krum accepted his power by drinking from his skull. With this, he did not humiliate the Emperor; on the contrary, he acknowledged Nicephorus's power and wished it to be passed to himself by drinking from his skull. Evidently, Krum did not share Theophanes' opinion that Nicephorus was an incompetent commander leading a riff-raff army; quite on the contrary, Krum thought highly of the strength of the Byzantine army and the military ability of Nicephorus. As is seen by Krum's repeated humble peace proposals, he did not underestimate even for a moment Nicephorus as his adversary. There is no evidence for Krum making drinking cups from the heads of other commanders that he defeated: the Avar khagan and Michael Rangabe; probably he did not consider them great enough for these rites.
Location of catastrophe
The following objections can be raised against the opinion that Kotel Pass was the location of the battle: First of all, it is too risky to rely on local legends for determining the location of historic events, if those are not supported, at least in part, by literature data. This precaution is necessary especially with the issue at hand, first, because such legends for Nicephorus' defeat exist in many places throughout Eastern Bulgaria (around Shumen and Preslav), not only among Bulgarian but also among the Turkish population there, and second, because those legends cannot be considered to go back to old times: they were created relatively recently, during Bulgarian Renaissance and rediscovery of Bulgarian history. This is best exemplified by the name "Greek Hollow". This name in the mouth of old Kotel citizens sounds "Grăshki" and according to some "Grishki" or "Grashki" (=Pea Hollow), or even as in Bury, "Groshki" (=Penny Hollow) so that etymology can have completely different meaning.
Without doubt, however, the best evidence can be found in the chronological data in Theophanes' account. As we saw above, Nicephorus entered the Bulgar territory through the border fortress Marcelae on July 20. The first 3 days he spent on the move in skirmishes with the Bulgars, and when he entered the mountain pass, he chose steep paths, so that on the fourth day, July 23, he could enter into the residence of the Bulgar Khan. One cannot believe the words of Theophanes that Nicephores plundered and killed the population of the town, and then burned Krums' palaces only in one day, and immediately went back; because, as we saw, Krum, even after the plunder, negotiated for peace, probably to gain time while blocking the entrances and the exits of the pass, which happened on the 5th and the 6th day (Thursday and Friday) while Nicephorus was still in Pliska. Evidently, he left on the 6th day because on the 7th day (Saturday) on July 26 at dawn the Bulgars were already attacking Nicephorus' tent. It is hardly conceivable that in such a short time the Byzantians would reach the peaks Vetrila and Vid in the Kotel Pass and take good strategical positions, and Nicephorus make a military camp in the locality "Karenika" in the Kotel Pass. Moreover, Nicephorus learned about the Bulgar fortifications while he was on the move and was already inside the pass, and this happened in the night of the 7th day, because if he knew before that he wouldn't want "to have wings" but would seek another way to retreat. The confusion and panic in the Byzantine army show that it was attacked without warning, so that it is unconceivable that Nicephorus would have time to fortify and choose "important positions" and, in general, to prepare for battle. All this shows that the defeat of Nicephorus happened not far from Krum's residence and this can be in the Chalăka or the Vărbitsa Passes. It is hard to say which one; however, if we take into account that Nicephorus chose the shortest way for retreat, it is more probable that Nicephorus chose the Vărbitsa pass, through which he entered into Bulgaria. [9, p.58]
Khan Krum feasts after the victory over Nicephorus I Genik.  Inscription (in Old Bulgarian Slavic): "Krum Kniaz encased the head of tsar Nicephores and drank to the health of Bulgars."
For Bulgaria, this victory had tremendous importance: it not only saved it from the great threat from Byzantium and returned all the lands taken from them, but strengthened all Bulgar conquests in the West together with Serdika and secured them from future attacks by Byzantine emperors, for whom Bulgaria became a permanent threat. For a long time, until the reign of John I Tzimiskes (ca. 970), Byzantines were afraid to pass the Balkan Mountains. Krum had good reason to be exultant. The whole effect of Constantine Copronymus’ long campaigns had been wiped out in one battle. He could face the Empire now in the position of conqueror of the Emperor, on equal terms, at a height never reached by Isperih or Tervel. Henceforward he would not have to fight for the existence of his country; he could fight for conquest and for annexation. Moreover, in his own country his position was assured; no one now would dare dispute the authority of the victorious Khan. He could not have done a more useful deed to strengthen the Bulgar crown. Moreover, this victory elevated the image of the Bulgar Khan in the eyes of Macedonian Slavs and with this opened a way for extension of the Bulgar state to the southwest. This pride of Krum is most clearly evident in the story about Nicephorus' head:
As he cut the head of Nicephorus, Krum put it on a stake for several days to show it to the tribes coming to him to our disgrace. After that he took it, plated it with silver from the outside and proudly made the Slav knyazes [princes] drink from it. [1, p. 491]
Content with their victory, the Bulgars did not at once follow it up with an invasion. But late next spring (812) Krum attacked the Imperial fortress of Develtus, a busy city at the head of the Gulf of Burgas, commanding the coast road to the south. It could not hold out long against the Bulgars. Krum dismantled the fortress, as he had done at Serdika, and transported the inhabitants, with their bishop and all, away into the heart of his kingdom. In June the new Emperor Michael set out to meet the Bulgars; but the news that he was too late to save the city, together with a slight mutiny in his army, made him turn back while he was still in Thrace. His inaction and the Bulgar victories terrified the inhabitants of the frontier cities. They saw the enemy overrunning all the surrounding country, and they determined to save themselves as best they could. The smaller frontier forts, Probatum and Thracian Nicaea, were abandoned by their population; even the population of Anchialus (today Pomorie) and Thracian Berrhoea (today Stara Zagora), whose defences Empress Irene had recently repaired, fled to districts out of reach of the heathen hordes. The infection spread to the great metropolis-fortress of Western Thrace, Philippopolis (today Plovdiv), which was left half-deserted, and thence to the Macedonian cities, Philippi and Strymon. In these last cities it was chiefly the Asiatics transported there by Nicephorus that fled, overjoyed at the opportunity of returning to their homes. Over the next two years, Krum was able to attack the empire in the vicinity of Constantinople itself, although he was never able to take the city. Michael attempted to recover from the loss, but was defeated in 813 at the Battle of Versinikia; the danger did not subside until Krum himself died in 814.
2. Scriptor Incertus. Anonymous Vatican Narration (Narratio anonyma e codice Vaticano), In: Codice Vaticano graeca 2014 (XII s.) ff. 119-122; Ivan Duychev (1936) New Biographic Data on the Bulgarian Expedition of Nicephorus I in 811, Proc. Bulg. Acad. Sci. 54:147-188 (in Bulgarian); H. Grégoire (1936) Un nouveau fragment du "Scriptor incertus de Leone Armenio", Byzantion, 11:417-427; Beshevliev, V (1936) The New Source About the Defeat of Nicephorus I in Bulgaria in 811, Sofia University Annual Reviews, 33:2 (In Bulgarian).
3. Mannases Chronicle, 1335-1340. Apostolic Library. The Vatican.
4. Michael the Syrian, Chronique de Michel le Syrien, Patriarche Jacobite d'Antioche (1166-1199), published by Jean Baptiste Chabot (in French). 1st Ed. Paris : Ernest Leroux, 1899-1910, OCLC 39485852; 2nd Ed. Bruxelles: Culture et Civilisation, 1963, OCLC 4321714
5. B. Flusin (trans.), J.-C. Cheynet (ed.), Jean Skylitzès: Empereurs de Constantinople, Ed. Lethielleux, 2004, ISBN 2-283-60459-1.
7. Bozhilov, Ivan, and Gyuzelev, Vasil. 1999. History of Bulgaria. Vol. 1: History of Medieval Bulgaria 7-14 c. AD. Anubis Publishing, Sofia, ISBN 954-426-204-0. (in Bulgarian)
8. Zlatarski, Vasil N. 1918 (in Bulgarian). Medieval History of the Bulgarian State, Vol I: History of the First Bulgarian Empire, Part I: Age of Hun-Bulgar Domination (679-852). Sofia: Science and Arts Publishers, 2nd Edition (Petar Petrov, Ed.), Zahari Stoyanov Publishers, 4th Edition, 2006. ISBN 9547366284.
11. Bury, J.-B. (1912). A History of the Eastern Roman Empire from the fall of Irene to the accession of Basil I (802—867). Macmillan & Co., Ltd., London. ASIN B000WR1S6Q, OCLC/WorldCat 1903563.
13. István Bóna, Southern Transylvania under Bulgar Rule, Chapter II.6 In: History of Transylvania (Béla Köpeczi, Gen. Ed.), Vol. 1, 2001-2002 Social Science Monographs, Boulder, Colorado; Atlantic Research and Publications, Inc. Highland Lakes, New Jersey
16. This is the period of the year when Sirius first becomes visible above the eastern horizon at dawn, after a period when it was hidden below the horizon or when it was just above the horizon but hidden by the brightness of the sun. The period of the heliacal rising of the Dog Star determines the Dog Days, or as the Romans called them, caniculares dies (days of the dogs). For the ancient Egyptians, Sirius appeared just before the season of the Nile's flooding, so they used the star as a "watchdog" for that event on which they based the Egyptian calendar.
17. Brady’s Clavis Calendarium, 1813
18. For the ancient Greeks, the appearance of Sirius heralded the hot and dry summer. Due to its brightness, Sirius would have been noted to twinkle more in the unsettled weather conditions of early summer. The traditional ancient timing of the Dog Days is the 40 days beginning July 3 and ending August 11; however, at present, due to the precession of the equinoxes, the heliacal rising of Sirius has shifted with 37 days towards the end of the year so that it begins on August 9 and ends on September 17.
19. The image was made with the help of the astronomy software Home Planet, release 3.3a, with Pliska coordinates 43°23′N 27°8′E. Half of the Sun's disk has appeared above the horizon from the east. The Dog Star (Sirius of the constellation Canis Major (Big Dog)) has risen 2 minutes before the Sun (heliacal rising). The moon is late in its last quarter in the constellation Cancer (Crab).
20. Theophanes (p. 486) gives the exact day in which the expedition set out; however, the original text is damaged so that only the month is legible.
21. Blasius Kleiner (1761) History of Bulgaria (in Latin), translated in Bulgarian by Karol Telbizov, edited by Ivan Duychev, Bulgarian Academy of Sciences Publishing House, Sofia 1977
22. Todorov Dzh.Zh., Stoyanov R.I., Ivanov I.R., and Chalakov I.H. The Lost Town: A History of Sadovo (in Bulgarian)
23. Distance was measured using the distance measuring tool of the free software application Google Earth, version 4.2
24. John Haldon. The Organisation and Support of an Expeditionary Force: Manpower and Logistics in the Middle Byzantine Period. In: Byzantium at War (Edited by Nicolas Oikonomides), Athens: Institute for Byzantine Studies, 1997
25. Georgius Monachus. Chronicon, p.774
26. Anastasius Bibliothecarius. Chronographia tripertita, p.329
27. Calculated with the freeware program Home Planet, v. 3.3a, Sun/Moon info module.
28. А. Васильев, Арабский синаксарь о болгарском походе императора Никифора I. В „Новый сборникъ” статей в честь проф. В. И. Ламанского, Петроград, 1905, стр. 361—362.
29. Шкорпил В. и К., Някои бележки върху археологическите и историческите изследвания в Тракия, Пловдив 1885.
30. K. Шкорпил. Материалы для болгарских древностей Абоба-Плиска. Известия Русского Археологическото Института в Константинополе, Х (1905).