The game sends an infinite amount of troops after the alarm has been set. The best way to defend yourself is to charge ahead as fast, but also as safely, as possible. In some levels you can cut down the number of men by almost fifty. If you are low on ammunition, health, etc., this is a good way to cut down on the amount of men you have to deal with. It also works sometimes if you send a man far ahead and hold him there, then do your objectives for your mission.