Cheat mode:
At the main menu, press X, X, Y, Y, click Left Analog-stick, click Left Analog-stick, click right Analog-stick, click right Analog-stick, L, L, R, R. There is no confirmation message. Start a game, then pause and enter the options screen. The "Cheats" option will be unlocked with Trooper Level, Enemy AI, and Mission List selections.

Thanks to Revolution readers Craig Kubicek, shawn azarafza, Luis Martinez, and efrain!