Adding Some Difficulty

[jivemi]

It wouldn't work with campaigns, but sticking with advance or assault scenarios usually does the trick.

[PvtJoker]

For long campaigns:

Like Andy said, Italy can be pretty challenging. Start against the French (have fun with CV-35s against Char B1s/B1bis) and then switch to North Africa. Stick to historically available tanks for added challenge (CV-35 for 1940 with just a couple of M-11/39). In 1941 it will get a little easier, but in 1942 very interesting again. Don't buy heavy artillery for anything else than assault missions (which is also historically accurate for almost any country).

Nationalist China is a good choice as well. Stick to almost pure infantry force. On the other hand Japan can get fairly challenging in 1943-1945, especially with historical OOB; mostly light tanks and tankettes with the Shinhoto Chi-Ha as the best tank available (the newer tanks never left the home islands).