Defend is very similar to the mission Attack were you must find and attack a fire base and then hold of the counter attack. If any one of the enemy reaches the center of the base you the mission is a failure. After a certain amount of time you will get a prompt telling you that you need to kill X amount of hostiles around the base and then you can leave. Be careful ,even if you get the prompt You can still lose if a hostile reaches the center. Mines, grenades, heavy weapons, and fire support (specifically gun ships) are all recommended. Soldiers with a high combat and spot skill are recommended. The local defenses will usually not be very heavy if you have a low heat score ,but SPG-9s, mortars, and DShK's are all possibilities.