In 1096, Tancred joined his maternal uncle Bohemond on the First Crusade, and the two made their way to Constantinople. There, he was pressured to swear an oath to Byzantine Emperor Alexius I Comnenus, promising to give back any conquered land to the Byzantine Empire. Although the other leaders did not intend to keep their oaths, Tancred refused to swear the oath altogether. He participated in the siege of Nicaea in 1097, but the city was taken by Alexius' army after secret negotiations with the Seljuk Turks. Because of this, Tancred was very distrustful of the Byzantines.