Ivan Asen I of Bulgaria

The Byzantine themes (or districts) of Bulgaria and Paristrion

A special tax, levied to finance the Emperor's marriage to Margaret of Hungary had brought the Bulgarian and Vlach population to the edge of an uprising before the public humiliation of Asen and his brother at the imperial camp.[17][23] In spite of the general discontent, the brothers were initially unable to stir up a rebellion, because their compatriots did not believe they had any chance against the imperial troops.[24] Theodor and Asen took advantage of the sack of Thessaloniki by the Normans, during which icons of Demetrius of Thessaloniki, the patron saint of the town, were taken to Bulgaria.[25] They built a "house of prayer"[26] and summoned Bulgarian and Vlach shamans to the site.[21][25] The brothers instructed these "demoniacs", as Choniates called them, to declare before the mob that God "had consented to their freedom" and Saint Demetrius would "come over to them" from Thessaloniki "to be their helper and assistant"[26] against the Byzantines.[27][28]

Theodor was crowned and assumed the name Peter, thus adopting the name of a 10th-century tsar (or emperor) of Bulgaria.[29][30] The coronation and Theodor's new name are evidence the brothers wanted to demonstrate from the beginning they had established a state which was the political successor of the First Bulgarian Empire.[31] They laid siege to Preslav, the old capital of the Bulgarian Empire but could not capture it.[29][32] During the first months of 1186, the brothers made plundering raids against Thrace, seizing captives and cattle.[28][32] Isaac II led a counteroffensive against the rebels in person, but they resisted the invaders hiding in "inaccessible places"[33] in the mountains.[28] It was only the solar eclipse of 21 April 1186 that enabled the imperial troops to mount an unexpected attack and defeat the rioters.[28] Peter and Asen fled from their homeland and crossed the Lower Danube, seeking military assistance from the Cumans.[28]

Isaac II thought his victory was decisive and returned to Constantinople without securing the defense of Paristrion.[34] Peter and Asen made an alliance with some Cuman chieftains who helped them to return to the Byzantine Empire in the autumn.[31][34][35] Choniates wrote contradictory reports about the negotiations between the brothers and the Cumans.[31] In a formal speech, he attributed the alliance to Peter's efforts; in his chronicle, he emphasized Asen's role.[31] Shortly after their return, the brothers took control of Paristrion and launched a plundering expedition against Thrace.[36] Asen's military tactics—the application of sudden raids and quick withdrawals—prevented the imperial troops from making successful counterattacks.[37] Choniates emphasized the brothers were not simply content to seize Mysia (or Paristrion) but had decided to "unite the political power of Mysia and Bulgaria into one empire as of old",[38] referring to their attempt to restore the First Bulgarian Empire.[34]

Second Bulgarian Empire from 1185 to 1196, according to a Bulgarian historical atlas. The theory that Bulgaria included Oltenia and Muntenia, as it is presented on the map, is not universally accepted by historians.[39]

Seals bearing the inscription Ivan "basileus" (or emperor) of the Bulgarians were found in Constantinople and other places.[40] According to George Akropolites, "Asen ruled over the Bulgarian race as emperor for nine years" before he died in 1196.[41] This suggests Asen became his brother's co-ruler in 1187 or 1188.[42][43] The Byzantines launched a series of unsuccessful campaigns against the rebellious Bulgarians and Vlachs, but they could not prevent Peter and Asen from securing their rule in Paristrion.[44] Isaac II personally led his troops against the brothers' realm and laid siege to Lovech in the spring of 1188.[45] Although he could not occupy the fortress, the Byzantines captured Asen's wife, Helen, and his younger brother, Kaloyan.[46][47] He was held hostage in Constantinople for years.[46]

The arrival of the crusader army of the Holy Roman Emperor, Frederick Barbarossa, in the Balkan Peninsula in July 1189 enabled Peter and Asen to occupy new territories of the Byzantine Empire.[48][49] One of the chronicles of Barbarossa's crusade, The History of the Expedition of the Emperor Frederick, explicitly mentions that they seized "the region where the Danube flows into the sea"[50] (present-day Dobruja) and parts of Thrace.[51] When writing of the negotiations between Barbarossa and the brothers' envoys during the march of the crusaders across the Balkans, primary sources mention only Peter, suggesting he was regarded as the senior ruler of Bulgaria.[52] The crusaders left the Balkans for Asia Minor in March 1190.[53]

Shortly after the crusaders' departure, Isaac II Angelos moved into the lands under the rule of Peter and Asen.[53][54] He could not defeat the Vlachs and Bulgarians, because they avoided a pitched battle, forcing the Emperor to start to retreat.[55] The imperial army was ambushed and defeated at a mountain pass.[56] The victorious Vlachs and Bulgarians, along with their Cuman allies, launched new raids against Thrace, pillaging Anchialos (now Pomorie in Bulgaria) and other towns.[57] Isaac II defeated the Cumans near Adrianople (now Edirne in Turkey) in April 1191.[58] Thereafter his cousin, Constantine Angelos Doukas, routed Peter and Asen's troops in a series of battles.[59]

A eulogy delivered in praise of Isaac II in 1193 referred to Asen as a "reckless and obdurate rebel", surrounded by "imperial traps", while describing Peter as a "stumbling block" and an "adverse wind" to his brother.[60][61] The speech shows, Byzantine intrigues stirred up a conflict between the brothers in 1192.[62] Madgearu says, Peter was allegedly willing to make peace with the Byzantines, but Asen wanted to continue the war.[62] Akropolites knew that Peter moved from Tarnovo to Preslav at an unspecified date, a region known as "Peter's land"[41] even in the 13th century.[62] Historians Madgearu[62] and Paul Stephenson[60] agree, the sources provide evidence the brothers divided their realm around 1192, with Asen retaining Tarnovo and its region.

After Constantine Angelos Doukas was blinded during the rebellion against Isaac II, the Vlachs and the Bulgarians resumed their attacks against the Byzantine Empire.[63][64] The Emperor dispatched Alexios Gidos and Basil Vatatzes to wage war against the invaders, but their united armies were almost annihilated in the Battle of Arcadiopolis.[64] Peter and Asen conquered new territories in Thrace, including Plovdiv.[65]

Isaac II decided to launch a new campaign to recover Thrace himself.[65] While he was mustering his troops at Kypsela, his brother, Alexios, captured and blinded him on 8 April 1195.[66] Alexios III sent envoys to Peter and Asen, proposing to make peace with them.[66] The brothers refused the new emperor's proposal.[66] Asen moved into Byzantine territory and defeated Alexios Aspietes.[66] He captured the Byzantine fortresses along the River Struma leaving Vlach and Bulgarian troops to garrison them.[66][67]

A new Byzantine army, under the command of the Emperor's son-in-law, Isaac Komnenos, launched a counterinvasion.[66][68] Asen's Vlach, Bulgarian and Cuman troops surrounded the invaders and defeated them near Serres.[67] Komnenos was captured by a Cuman warrior who secretly tried to hold him to ask for a huge ransom from the Emperor. When Asen was informed of Komnenos's capture, he ordered the Cuman to hand over his captive.[68]

A boyar Ivanko stabbed Asen in 1196, but the motive for this act is uncertain.[66] Choniates, who narrated the events, recorded two versions.[66] According to one account, the captive Isaac Komnenos persuaded Ivanko to kill the Tsar, promising to give his daughter in marriage to him.[69] The second version claims, Ivanko had "clandestine sexual relations with the sister of Asen's wife",[70] but their affair was revealed to Asen.[67][71] He decided to have his sister-in-law executed for the illicit love affair which insulted his family, but his wife persuaded him to punish Ivanko instead of her sister.[71] Asen ordered Ivanko to come to his tent late at night. Ivanko who had been informed about the tsar's decision came with a sword hidden under his garments.[67][71] He killed Asen during the meeting.[71]

Choniates stated, Ivanko wanted to rule "more justly and equitably" than Asan who had "governed everything by the sword".[72][73] Stephenson concludes, Choniates' words show that Asen had introduced a "reign of terror", intimidating his subjects with the assistance of Cuman mercenaries.[73] Vásáry, however, says the Byzantines encouraged Ivanko to kill Asen.[74] Ivanko attempted to assume control in Tarnovo with Byzantine support, but Peter forced him to flee to the Byzantine Empire.[74] Peter charged Kaloyan with the governing of Asen's realm.[74]