War on Terror

The War on Terror is an ongoing military campaign, conducted by several NATO-coalition nations, that is focused on the Middle East with the intent to eliminate both terrorist organizations and the government regimes that support them. The war began in late 2001, after the terrorist attacks on 11 September of that year.

Sometime during the conflict, the Assassin Emmanuel Barraza was tasked by the Assassin leader William Miles with finding the Templars' presence behind the war. During the mission, Barraza was forced to eliminate a bus full of bomb-rigged children, leading him to vow to never again take another human life.

Reference

 * Assassin's Creed: Initiates - "Dishonorable Discharge"