The main historical storyline of the first Assassin's Creed is set in 1191.
That date places Altaïr Ibn-La'Ahad's missions during the Third Crusade, a period when Crusader and Muslim forces contested territory in the eastern Mediterranean. The game presents fictionalized versions of historical cities including Masyaf, Jerusalem, Acre, and Damascus.
Altaïr is a member of the Assassin Brotherhood who is demoted after a failed mission. He must regain his standing by investigating and assassinating targets connected to the wider conflict. The historical sequences are experienced by Desmond Miles through the Animus in the modern-day storyline.
The year is sometimes confused with 1099, when Jerusalem was captured during the First Crusade, or 1291, the year Acre fell. Neither is the main date used for the first game's historical campaign.