I think that this is the one indispensible resource for the miniatures wargamer. This Encyclopedia of Military History covers virtually every armed conflict since 3600 BC. Whenever I think about starting a new period, this is the book I turn to first. While it doesn’t cover any period in the detail that a grognard would demand, it does offer enough names, dates and places to satisfy your curiosity, or settle a bet. It occupies a prime spot on my shelf.
The Harper Encyclopedia of Military History: From 3500 B.C. to the Present