Thomas the Slav (Greek: Θωμᾶς, romanized: Thōmas, c. 760 – October 823) was a 9th-century Byzantine military commander, most notable for leading a wide-scale revolt in 821–23 against Emperor Michael II the Amorian (r. 820–829).

An army officer of Slavic origin from the Pontus region (now north-eastern Turkey), Thomas rose to prominence, along with the future emperors Michael II and Leo V the Armenian (r. 813–820), under the protection of general Bardanes Tourkos.

Thomas quickly secured support from most of the themes (provinces) and troops in Asia Minor, defeated Michael's initial counter-attack and concluded an alliance with the Abbasid Caliphate.

Thomas's rebellion was one of the largest in the Byzantine Empire's history, but its precise circumstances are unclear due to competing historical narratives, which have come to include claims fabricated by Michael to blacken his opponent's name.

According to the first account, Thomas first appeared in 803 accompanying general Bardanes Tourkos, and pursued a military career until launching his revolt in late 820.

Nevertheless, the French Byzantinist Paul Lemerle came to consider it an unreliable later tradition created by his rival Michael II to discredit Thomas, and rejected it altogether, preferring to rely on the first account alone.

According to a later hagiographical tradition, before launching his revolt, Bardanes, in the company of his three young protégés, is said to have visited a monk near Philomelion who was reputed to foresee the future.

[12] Bury suggests that he fled (for a second time according to his interpretation) to the Arabs,[13] a view accepted by a number of other scholars, such as Romilly James Heald Jenkins.

[20] Some recent studies follow Lemerle and prefer the account of Symeon Logothetes—generally considered the most accurate of the 10th-century sources[21]—according to which Thomas rebelled a few days after the murder of Leo and in reaction to it.

Michael, on the other hand, was virtually unknown outside the capital; his military record was unremarkable, he was uneducated and coarse of manner, his stutter earned him ridicule, and he was reputed to sympathize with the heretical religious sect of the Athinganoi, to which his family had belonged.

[25] Byzantine accounts of Thomas's rebellion state that he did not in fact claim the throne under his own name but assumed the identity of Emperor Constantine VI, who had been deposed and murdered by his mother, Irene of Athens, in 797.

Nevertheless, the ambiguous phrasing of the sources, the iconoclast leanings of many themes in Asia Minor, and Thomas's alliance with the Arabs seem to speak against any open commitment to icon veneration on his part.

[31] The account of Theophanes Continuatus on Thomas's revolt states that in this time, "the servant raised his hand against his master, the soldier against his officer, the captain against his general".

This has led some scholars, chiefly Alexander Vasiliev and George Ostrogorsky, to regard Thomas's revolt as an expression of widespread discontent among the rural population, which suffered under heavy taxation.

[34] Genesios and other chroniclers further state that Thomas won the support of "Hagarenes, Indians (Zutt),[35] Egyptians, Assyrians, Medians, Abasgians, Zichs, Iberians, Kabirs, Slavs, Huns, Vandals, Getae, the sectarians of Manes, Laz, Alanians, Chaldians, Armenians and every kind of other peoples".

[36] This has led to modern claims that Thomas's rebellion represented an uprising of the empire's non-Greek ethnic groups,[1][37] but according to Lemerle, this exaggerated account is yet another piece of hostile disinformation.

It is almost certain, however, that Thomas could count on support among the empire's Caucasian neighbours, for the presence of Abasgians, Armenians, and Iberians in his army is mentioned in the near-contemporary letter of Michael II to Louis the Pious.

The reasons for this support are unclear; Thomas may have made unspecified promises to their rulers, but Lemerle suggests that the Armenians might have in part been motivated by revenge for Leo, their murdered kinsman.

In exchange, Thomas is said to have promised to cede unspecified territories and become a tributary vassal of the Caliph, though the agreement's exact terms are left unclear in the primary sources.

[31] Meanwhile, Michael II tried to win support among the iconophiles by appointing a relative of his as Archbishop of Ephesus, but his plan failed when the latter refused to be consecrated by the avowedly iconoclast Patriarch Antony I Kassimates.

[48] Anticipating Thomas's move, Michael had gone out at the head of an army to the themes of Thrace and Macedonia in Constantinople's European hinterland and strengthened the garrisons of several fortresses there to secure the loyalty of their populace.

Facing no opposition from the imperial fleet, the rebels broke or unfastened the chain and entered the Golden Horn, taking station near the mouths of the Barbysos river, where they awaited the arrival of Thomas and his army.

Thomas's army viewed the plea as a sign of weakness, and advanced confidently to begin the assault, but as they neared the wall, the defenders opened the gates and attacked.

[61] Thomas was unable to resume the siege: aside from the heavy casualties his army likely suffered, his fleet, which he had left behind in the Golden Horn, surrendered to Michael during his absence.

Thomas marched to meet them and planned to use a stratagem to outwit his opponents: his men, ostensibly demoralized, would pretend to flee, and when the imperial army broke ranks to pursue them, they would turn back and attack.

His strategy was motivated by the political and propaganda expedient of appearing merciful—"in order to spare Christian blood", as Michael himself put it in his letter to Louis the Pious—but also, according to the chroniclers, by fear of demonstrating to the Bulgarians that the Byzantine cities' fortifications could fall to attack.

[65] In Asia Minor, Thomas's partisans hoped to lure Michael away by allowing the Arabs free passage to raid the provinces of Opsikion and Optimaton, which were loyal to the emperor.

Lemerle holds that several factors played a role in his defeat: the Asian themes he did not subdue supplied reinforcements to Michael; Thomas's fleet performed badly; and the Bulgarian offensive diverted him away from the capital and weakened his army.

[73] Thomas's rebellion was the "central domestic event" of Michael II's reign,[74] but it was not very destructive in material terms: except for Thrace, which had suffered from the prolonged presence of the rival armies and the battles fought there, the larger part of the empire was spared the ravages of war.

[75][77] This is traditionally held to have resulted in a military weakness and internal disorder which was swiftly exploited by the Muslims: in the years after Thomas's rebellion, Andalusi exiles captured Crete, and the Aghlabids of Ifriqiya (modern Tunisia) began their conquest of Sicily, while in the East, the Byzantines were forced to maintain a generally defensive stance towards the Caliphate.