His rule
Thoros I died in 1129 (or in 1130), and his son Constantine II died a few months later, in the course of a palace intrigue.[1] Other authors (e.g., Jacob G. Ghazarian, Vahan M. Kurkjian) suggest that Thoros I died without a male heir and was directly succeeded by Leo.[2][3]
Conflicts with the Franks
In February 1130, Bohemond II, Prince of Antioch, whose ambition was to restore his principality, thought that the moment had come to recover Anazarbus (a former Antiochene town which had fallen into the possession of Thoros I).[1] He marched with a small force up the river Jihan towards his objective.[1] Leo was alarmed and appealed for help to the Danishmend emir, Ghazi.[1] As Bohemond II progressed carelessly up the river, meeting only light resistance from the Armenians, the Danishmend Turks fell on him and massacred the whole of his army.[1] However, it was due to Byzantine intervention that the Turks did not follow up their victory; and Anazarbus remained in Armenian hands – Michael the Syrian says that John II Comnenus at once started an offensive against the Turks.[1]
Soon after Bohemond II’ death, Leo protected in his rear by an alliance with the Danishmend emir, descended into the plain; after a brief unsuccessful siege of Seleucia,[2] he seized the three cities of Mamistra, Tarsus and Adana in 1131.[1] In 1133, Leo captured Sarventikar, on the slopes of the Amanus Mountains, from Baldwin of Marash.[1] But the Armenian hold over Cilicia was weak: bandits found refuge there, and pirates hung about its coasts.[1]
In 1136, the new prince of Antioch, Raymond I decided that his first action must be to recover Cilicia.[1] With the approval of King Fulk of Jerusalem he marched with Baldwin of Marash against Leo.[1] But Leo, with the help of Count Joscelin II of Edessa (who was his nephew), drove back the Antiochene army.[1] Triumphant, Leo agreed to have a personal interview with Baldwin of Marash, who treacherously made him prisoner and sent him off to captivity in Antioch.[1]
In Leo’s absence his three sons quarreled: the eldest, Constantine, was eventually captured and blinded by his brothers.[1] Meanwhile, the Danishmend emir, Mohammed II ibn Ghazi, invaded Cilicia, destroyed the harvest.[1] Shaken by these disasters, Leo bought his freedom by offering to give up the Cilician cities (Sarventikar, Mamistra and Adana)[3] to Raymond I;[1] in addition he paid 60,000 gold pieces and gave his son as a hostage;[3] but on his return home he forgot his promise.[1] A desultory war broke out again, till, early in 1137, Joscelin II patched up a truce between the combatants.[1] An alliance was then formed against the Emperor John II Comnenus, who was then pressing his claims against Antioch as well as Cilicia.[3]
The (re-)occupation of Cilicia by the Byzantines
In the spring of 1137, the imperial army, with the Emperor and his sons at its head, assembled at Attalia (today Antalya in Turkey) and advanced eastward into Cilicia.[1] Leo moved up in an attempt to check its progress by taking the Byzantine frontier fortress of Seleucia, but was forced to retire.[1] The Emperor swept on, past Mersin, Tarsus, Adana and Mamistra, which all yielded to him at once.[1]
Leo relied on the great fortifications of Anazarbus to hold him up.[1] Its garrison resisted for 37 days, but the siege engines of the Byzantines battered down its walls, and the city was forced to surrender.[1] Leo retreated into the high Taurus Mountains, while the emperor led his forces southward into the plain of Antioch.[1]
After the emperor had asserted his authority over the Principality of Antioch, he returned to Cilicia to finish off its conquest. The family castle of Vahka (today Feke in Turkey) held out for some weeks.[1] Eventually, John invited Leo to a meeting under a false promise of peace, where the prince was captured. Leo and two of his sons, Roupen and Thoros, were subsequently taken prisoner.[4]