Cheat Menu
Enter the following at the main menu:

L1, R1, L1, R1, SQUARE, TRIANGLE, CIRCLE, TRIANGLE, SQUARE, CIRCLE - Cheat Menu

Cnflict vietnam
L1,R1,L1,R1,SQUARE,TRIANGLE,CIRCLE,SQUARE,CIRCLE

When spipping the dot wont move
When you are sniping try not to move also try to hit the guy this will make your stars go up fast

Invisible
R1,triangle,left,right,up

Very Helpful Cheats to learn the game fast!!
Tips
rating: ---1st Air Cav mission
At the beginning of the level, get all the ammunition in the weapons room (to your left at the very start). Then, move on to the Major. When he tells you to go to the bunkers, take Ragman and the Doc and crouch down next to the Major. Then, take Junior and put him in the first bunker to your right and up the stairs. Then, take Hoss and put him in the next bunker down. Whenever someone dies, the Doc will heal them.
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Tips:

Master cheat
On the main menu..

L1, R1, L1, R1, SQUARE, TRIANGLE, CIRCLE, TRIANGLE, SQUARE, CIRCLE

This will unlock a cheat option on the main menu with.. Unlimited ammo, Unlimited saves, Movie and level select.

Wouldn't advise unless you're REALLY stuck, spoils a good (but short) game

Easy Way To Beat Last Level (multiplayer)
Get player 1 to lob smoke grenades at the tank while player 2 gets in the drivable tank and blows up the tank.

Conflict: Vietnam (PS2)
Invincibility:At main menu enter R1,L2,R1,L2 to get invincibility for the everyone except the enemy.