There are a total of 9 nation specific units that you may use throughout the game.
Grand Cannon (France)
This gun is massive - it's actually a structure. Its range and power are excellent. It kills all units with one shot if it hits them. This structure is extremely useful for base defence. It is extremely effective against all armoured units - enemies will tend to try and take out your power to turn them offline.
Sniper (Great Britain)
These guys are equipped with long range rifles designed to take out infantry units. These guys never miss - click on an enemy infantry unit and say goodbye. They kill all infantry (excluding tesla troopers & desolators) units with just one shot! Try combining them with IFV's - an excellent deterrent to the desolator.
Once you have built an air force command you will get one deployment of these guys every few minutes - you tell the drop plane where to drop them off and within about 15 seconds about 5 GIs will land there. If the plane carrying them is shot down before they have been dropped then they all die. These guys are normal GI's - you just get to drop them wherever you want - which can be very useful if your enemy has weak air defences..
Tank Destroyer (Germany)
These tanks use special technology to pierce enemy armour. They are very effective against armoured vehicles - like tanks (as you may have guessed from the name). However these units are vulnerable to infantry attack and weak against structures.
Black Eagle (Korea)
This unit is a stronger adaptation of the harrier - it is quicker, more accurate and boasts a longer life bar. These units are very effective; but usually only in groups - best used for taking out lone tanks or important structures.
Tesla Tank (Russia)
This tank fires a tesla charge rather than shells - it is very useful; sporting the ability to kill enemy infantry instantly and the ability to decimate structures quickly. The tesla charges are also able to loop over walls. Used in groups these units are an extremely powerful force.
These units are designed especially for destroying the enemy base. Terrorists carry C4 charges wrapped around their bodies - they then 'kamikaze' - committing suicide in order to blow up enemies quickly. These units are very useful for obliterating structures quickly - but they are very vulnerable from attack; so make sure they are guarded.
Demolition Truck (Libya)
Another suicide unit - packed with explosive nuclear material. The idea of it is to take it into the enemy's base - then blow it up. The explosion destroys almost everything in a wide area around the truck and leaves signifcant amounts of radiation.
An excellent unit; it uses a radiation cannon to irradiate the ground around it - killing any enemy infantry that come nearby, as well as light armoured vehicles (such as the mirage tank). He also sports a zap gun - capable of killing enemy infantry in one shot.