Isaac Komnenos or Comnenus (; – after 1146), was the third son of Byzantine Emperor John II Komnenos by Irene of Hungary. He was bypassed by his father in favour of his younger brother Manuel I Komnenos for the succession, leading to a tense relationship between the two brothers after. He participated in the campaigns of his father and brother in Asia Minor, and was a fervent adherent of Patriarch Cosmas II of Constantinople, but little else is known about his life. Early life Isaac Komnenos was born as the third son of John II Komnenos, emperor of the Eastern Roman Empire (), and Irene of Hungary. When his oldest brother Alexios was crowned co-emperor in 1122, Isaac, along with his other brothers, was awarded the rank of sebastokrator by his father. He was a tall and imposing man, but, according to the Byzantine sources—who are admittedly partial to his youngest brother and eventual successor to John II, Manuel I Komnenos ()—prone to sudden outbursts of anger and harsh punishments, so that he was not much loved. According to John Kinnamos, Isaac participated in the 1136 campaign against Armenian Cilicia, where, during the siege of Anazarbos, he counselled his father to dress the wooden siege engines with bricks, so as to thwart the defenders, who were throwing heated irons to set them on fire. This stratagem allowed the Byzantines to capture the city. In 1142, while taking part in another campaign by John II in southern Anatolia, both Alexios and then the second brother, Andronikos, unexpectedly died. Isaac, who had also taken part in the campaign and sent to escort Alexios' corpse along with Andronikos, accompanied the bodies of his brothers back to Constantinople. Succession The deaths of his older brothers left Isaac as the obvious candidate for the succession, but shortly before his death on campaign in Cilicia in April 1143, John II Komnenos designated his fourth son Manuel as his heir, bypassing Isaac. The background for this nomination is unclear; contemporary Byzantine historians suggest that Manuel was more qualified, and stress that primogeniture was not decisive in Byzantine tradition. The contemporary Latin historian William of Tyre, on the other hand, plainly states that the main factor was Manuel's presence with the army and his ability to lead them safely home, while Isaac was away in Constantinople. According to William, the powerful megas domestikos (commander-in-chief of the army), John