American Sniper tells the true story of U.S. Navy SEAL Chris Kyle, played by Bradley Cooper. Kyle, who served four tours in Iraq, remains the most lethal sniper in American history. In the trailer, Cooper, who gained some 40 pounds for the role, struggles to decide on the fate of his target: a little boy who has just received a grenade from his mother. Kyle was shot and killed at a shooting range in 2013 in Texas, some four years after being honorably discharged from the U.S. Navy. American Sniper also stars Sienna Miller, Luke Grimes and Sam Jaeger. The film opens on Dec. 25.