First, deal the cards, so that each player has half the deck. Players should leave their cards face down in a pile. Each player plays a card off the top of their stacks into the middle. The highest card wins both cards and they should be put back on the bottom of the winner's stack. The game continues in the same fashion until someone wins.
When both players play the same card there is a war. Players leave the matched cards on the table and each plays one card face down from their stacks and then one more card face up from their stacks. The higher of the last two cards played wins all six cards. If both of those cards match, then the war continues with two more cards from each side until finally one card is higher than the other. This person wins ALL the cards. If someone runs out of cards during a war then the other person wins.
Variation for multiple players:
If more than two players play you can do the following. Deal all the cards out as evenly as possible among all players. All players play to the middle and highest card wins the pool. If two or more players match cards then ALL players have a war and play two additional cards - one face down and another face up. Highest card wins all. Any player who runs out of cards is out and the game continues with the rest.