Which historical conflict provides the main backdrop for the original Assassin's Creed?

The Third Crusade provides the main historical backdrop for the original Assassin's Creed.

The game is set in 1191, during the conflict commonly called the Third Crusade. Altaïr Ibn-La'Ahad carries out missions across the Holy Land, including the cities of Jerusalem, Acre, and Damascus, while the story presents Assassin and Templar factions operating within the period's political struggle.

The Third Crusade began after the capture of Jerusalem by Saladin in 1187 and included the campaigns of European rulers such as Richard the Lionheart. Assassin's Creed uses real historical figures and locations, but its secret-society plot and supernatural technology are fictionalized elements.

The setting is sometimes confused with later games set in other historical periods, including the Renaissance, the American Revolution, and the Golden Age of Piracy. Those eras belong to different main entries in the franchise.