I think one pretty interesting strategy game to try out is the Total War series. I guess its pretty realistic in terms of controlling an army!
By the way, for those interested in a free space combat simulation game (with battles between large capital ships), check this out:
Freespace 2
It was a commercial game released in 1999, but the company later provided the source code. That means its free now
There are some gaming communities who continued work on it by updating the graphics and sound. You can download it here:
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