Winning the Game
The primary way of winning a game of Domain is by wiping your opponent out completely. You do this by removing your opponent's territories from play, either by destroying them in combat or some other means. Once a player has no territories in play, they lose the game immediately.
Sometimes you don't need to utterly destroy your enemy to win a war. When a player begins their turn with an empty deck, that player loses the game, regardless of how many territories they still control.