Cheat mode:
Successfully complete all objectives in the single mission dossiers. The following codes can now be enabled.

Invincibility:
Press Back and then press X(2), A, B, A during game play. A message will confirm correct code entry. Your first soldier will be invincible, but the remaining members of your platoon can still be harmed. Note: You cannot complete the game normally when this code is active, as maps and dossier items can no longer be unlocked.

Big heads:
Press Back then press A, X, B, Y, A during game play. A message will confirm correct code entry.

Chicken explosives:
Press Back then press X(2), Y, A, B during game play. A message will confirm correct code entry.

Paper mode:
Press Back then press B, A, X, Y, A during game play. A message will confirm correct code entry. Characters will be two dimensional in this mode.

Slow motion:
Press Back then and press Y(2), B, X, A during game play. A message will confirm correct code entry.

High pitched voices:
Press Back then press X, A, Y, B, X during game play. A message will confirm correct code entry.

Hint: Medals:

Purple Heart: Get wounded or killed in action.
Bronze Star: Get 15 kills
Silver Star: Get 20 kills
Distinguished Service Cross: Get 25 kills
Congressional Medal Of Honor: Get 30 kills