Complete the game on "Hard" to unlock the "Authentic" difficulty. Your enemies are now excellent shots and use supreme tactics to counter your moves.
Complete the game on "Normal" difficulty to unlock the infinite ammunition cheat, giving you an un-numbered supply of bullets for any weapon you choose to pick up.
Beat the game under the "Easy" difficulty to unlock the "Old Movie" mode, which allows the game to be played in black and white, like actual WW2 footage.
Complete the game on "Authentic" to unlock the "Super Squad" cheat. This cheat enables your squad to have extreme strength against most small arm fire, but grenades, fixed machine guns and German armor can still eliminate them.
Make a profile with the name "2ndsquad".