-The game sends an endless amount of troops after the alarm has been set. The best way to defend yourself is to charge ahead as fast and 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 great 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.
-When the alarm has been raised, an unlimited number of enemies will start originating from certain points in the level such as barracks, campsites, or sometimes level boundary lines. The only way to stop this flow of enemies is to find the point at which they are appearing from and "secure" it. In other words, you have to kill all the enemies in the area (or building) and remain there for a minute or two to make sure no more show up. This way, you can systematically shut down the enemy's "points of origin", then finnish the level even after the alarm has been raised.