Day of Defeat: Source (2005) - MobyGames

The classic WWII infantry multiplayer mod for Half-Life, updated with the Source engine and distributed exclusively through Steam. Along with a huge leap in general graphics and sound quality, Day of Defeat: Source also uses Half-Life 2's famous physics engine and HDR lighting.

The core gameplay is mostly unchanged from the original Day of Defeat. It still pits two units of WWII infantry against each other, often at maps depicting sites of historical skirmishes. Players may choose from specialized classes such as scouts, snipers, and heavy support gunners. Teamplay is extremely important; a unit of lone wolves with exceptional individual skill won't stand a chance against a unit of averagely skilled players who really support each other.