This is a tought one, as WWII is my primary interest, I try to read up on as much as I can. Napoleonic warfare, the Mongols and Ancient civilizations ( Romans, Greeks and the Egyptians ) would probably be the next to follow.

Though now that I think about it, I am a huge fan of the American civil war as well.

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The war against Russia will be such that it cannot be conducted in a knightly fashion. This struggle is one of ideologies and racial differences and will have to be conducted with unprecedented, unmerciful and unrelenting harshness. -Adolf Hitler