Combat was the first game for the Atari 2600.  When the Atari VCS (original name for the Atari 2600) first came out, it came with the game combat.  When the Atari system no longer included Combat, Combat was available for sale separately from the system.  Combat is probably the most common game for the Atari 2600.  You can get one in almost any game lot on ebay and if you buy a used Atari today, it will most likely still come with Combat.

The game itself is actually a pretty cool game.  It is two player only, so you have to have another Atari fan in order to play this game (or you have to have very good dexterity to play both sides yourself, but I wouldn't recommend trying).  You can choose to be tanks, bi-planes or jets in a variety of settings.  You can also select to have invisible vehicles.  You play the game until time runs out an whoever has shot the other player the most times wins.  Again, this game is very easy to find.  I wouldn't buy it by itself.  If you don't have this game already, just check out ebay and buy a lot of games that you would like to have.  Combat is usually one of the games in the lot.