Two factions are fighting over... something. The original reason has either been completely forgotten or reduced to a historical footnote. They've been fighting for so long that no one can remember anything except the war. Sometimes, both sides will have an infinite supply of troops via some Applied Phlebotinum. If by some miracle one side does manage to win it usually isn't portrayed as a good thing. Questions like "And Then What?" might come up -- they've become so used to war that they don't know what they will do now that there is no longer someone to fight. Examples of Forever War include: