pause the game and then enter up, down, up, down on the D-Pad. After that, press right, left, right, down, up, and right. Your character will be given extra armor.
Pause the game and then enter up, down, up, down on the D-Pad. After that, press up, right, left, down, down, and up. Your character will be invincible.
In the first level of the Market Garden campaign, if you can parachute into the windmill, you'll not only have a nice vantage point, but you'll find a scope upgrade for the M1 Garand.
Thompson drum upgrade:
In the first level, parachute onto the red flare near the searchlight on the left of the screen. Wax the solider there and pick up a 50 round drum for the Thompson.
Thanks to Revolution Reader Choorbs, drwfan19!