This is actually true to an extent. If manpower is no issue, inf+arty is most efficient fighting force.As much infantry as you can. Unfortunately, the game is much shallower than it looks. With any nation, you just overwhelm your enemies with infantry if you can get more of them.
Supplement inf with couple MOT to exploit breakthrough. INF win battles, but MOT do the finishers through overrun, and quick land grabs.
Tanks are optional. If you have more IC than manpower, feel free to use tank. They are expensive but powerful breakthrough units against infantry doom stacks.
MECH are pretty bad.
CAV is also pretty bad, you use it because you often start with some.
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