Tlass joined the Ba'ath Party at the age of 15, and met Hafez al-Assad when studying at the military academy in Homs. The two officers became friends when they were both stationed in Cairo during the period of 1958–1961 United Arab Republic merger between Syria and Egypt: while ardent Pan Arab nationalists, they both worked to break up the union, which they viewed as unfairly balanced in Egypt's favor. When Hafez al-Assad was briefly imprisoned by Nasser at the breakup of the union, Tlass fled and rescued his wife and sons to Syria.