Well, technically, just about any war game is AH because you can change historical outcomes of battles or campaigns.
My absolute favorite AH game is the computer game "Enigma - Rising Tide", a somewhat simplistic naval warfare game set in an alternate 1930's-1940's after a German WW1 victory. One can play as the USA, German Empire, or a strange quasi-union of Japan and Great Britain in a fairly well-developed three-way global power struggle. The best parts are fake newspapers which introduce missions which are pretty clever AH themselves. Good graphics and fun, but surprisingly hard and completely unrealistic.