To strengthen his position, he forged an alliance with one of the rump state—Nicaea—by marrying Irene, a daughter of Theodore II of the prominent Laskaris family.

Early in his reign, his army invaded Severin, Hungary which outraged Béla IV; this led Hungarian troops to capture Vidin, an important town of the Bulgarian Empire and also saw the Hungarians besieging the Lower Danube region, leaving northwestern Bulgaria to Rostislav Mikhailovich (Béla's son-in-law), who had claimed Bulgaria in the years prior.

[5] Pirivatrić[6] and other scholars[7][8] also say, Konstantin may have been a son or a nephew of the Bulgarian boyar John Tihomir, who controlled Skopje in the late 12th century.

[9] Konstantin Tih mounted the Bulgarian throne after the death of Michael II Asen,[10] but the circumstances of his ascension are obscure.

[1] Rostislav captured Vidin, Mitso held sway over southeastern Bulgaria, but none of them could secure the support of the boyars who controlled Tarnovo.

[12] The marriage also forged an alliance between Bulgaria and Nicaea,[10][12] which was confirmed one or two years later, when the Byzantine historian and official George Akropolites came to Tarnovo.

[15] Soon after he concluded a peace treaty with Ottokar II of Bohemia in March 1261, Hungarian troops stormed into Bulgaria under the command of Béla IV's son and heir, Stephen.

[15] They captured Vidin and besieged Lom on the Lower Danube,[14] but they were unable to bring Konstantin to a pitched battle, because he withdrew to Tarnovo.

[15] Konstantin's minor brother-in-law, John IV Laskaris, was dethroned and blinded by his former guardian and co-ruler, Michael VIII Palaiologos, before the end of 1261.

[10] Michael VIII's army had occupied Constantinople already in July, thus the coup made him the sole ruler of the restored Byzantine Empire.

[15][17] Mitso, who still held southeastern Bulgaria, made an alliance with the Byzantines, but another powerful nobleman, Jacob Svetoslav, who had taken control of the southwestern region, was loyal to Konstantine.

[18] Benefiting from a war between the Byzantine Empire, the Republic of Venice, Achaea and Epirus, Konstantine invaded Thrace and captured Stanimaka and Philippopolis (now Asenovgrad and Plovdiv in Bulgaria) in the autumn of 1262.

[21] After seizing Mesembria from Mitso, Glabas Tarchaneiotes continued his campaign along the Black Sea and occupied Agathopolis, Sozopolis and Anchialos (now Ahtopol, Sozopol and Pomorie in Bulgaria).

[24] One of his uncles was a prominent leader of the Golden Horde and he sent messages to him to persuade the Tatars to invade the Byzantine Empire with Bulgarian assistance.

[28] The united Tatar and Bulgarian armies launched a sudden attack against Michael VIII who was returning from Thessaly to Constantinople, but they could not capture the emperor.

[34] To prevent Bulgaria from joining the anti-Byzantine coalition, Michael VIII offered his niece, Maria Palaiologina Kantakouzene, to the widowed Konstantin in 1268.

[35] Konstantin broke into Thrace in 1271 or 1272, but Michael VIII persuaded Nogai, the dominant figure in the westernmost territory of the Golden Horde, to invade Bulgaria.

[42] Due to the expensive and unsuccessful wars, repeated Mongol raids, and economic instability (Constantine was the first Bulgarian ruler to mint his own coins on a vast scale), the government was faced with a revolt in 1277.

What is clear is that a swineherd or swine-owner named Ivaylo became a leader of the discontented and attracted many (presumably mostly lower-class)[45] followers, asserting his control over a significant area.