At the 'Menu' screen enter the following codes quickly and if you've entered them correctly you'll hear a confirmation sound and a message will appear at the bottom of the screen.

Unlock ALL Free Play Maps:
Press L, L, L, L2, R2, R2, R2, L, R, L, Square

Turn Off Fog Of War (See ALL Units):
Press L, R, L, R2, L2, R2, L2, L, L, R, R, Square.

Complete the following Campaign or Mission Battle to unlock the corresponding map.

Unlock Tutorial Map in Free Play:
Complete ALL Tutorial levels.

Unlock MBT Type 69 and WZ551 (China):
Complete Mission 2 in Mission mode.

Unlock Leopard 2A6 (Germany):
Complete Mission 3 in Mission mode.

Unlock F35B (America):
Complete Mission 4 in Mission mode.

Unlock 'First Battlefield' Map in Free Play Mode:
Complete Campaign Level: First Battlefield.

Unlock 'Graduation' Map in Free Play Mode:
Complete Campaign Level: Graduation.

Unlock Type 74 SP1 (Japan) in Free Play Mode:
Complete Campaign Level: Graduation.

Unlock Type 61 SP and Type 74 SP1 (Japan):
Complete Campaign Level 2: Graduation.

Unlock 'Recruit Training' Map in Free Play Mode:
Complete Mission 1 in Mission mode.

Unlock 'Capture the Base Map' in Free Play Mode:
Complete Mission 2 in Mission mode.

Unlock 'Long Bridge to Battle' Map in Free Play Mode:
Complete Mission 3 in Mission mode.

Unlock 'Faraway Enemy' Map in Free Play Mode:
Complete Mission 4 in Mission mode.