Ranks of Bronze- Ancient warfare combined with SF
The Belisarius series- Aliens tampering with Earths ancient history
The Man Kzin War Series is good.
And The Bolo series is wonderful, if you love armor warfare.
The Berserker series is good if you love space warfare.
There are some collections of military fiction edited/compiled by David Drake, The Military Dimension, and under subjects like- Armor, Infantry, Dreadnaughts and so on.