Day of Defeat: Source is set in World War II, specifically the European Theatre in 1944. Players join the forces of either the United States Army or the German Wehrmacht and compete against each other in a variety of game modes. Players select from one of six classes to play as, each with its own role within the team. Player characters cannot take much damage, and in some circumstances can be killed by a single bullet, forcing players to make use of cover to stay alive. When a player character dies, that player starts a short countdown for reinforcements. When the timer runs out, the player and any friendly players killed in that time respawn into the game at their insertion point as the next wave of troops.