An oldie-but-goodie, Risk II was implemented by Microprose, before they were taken over by Hasbro, passed onto Atari, and gutted... Still the best digital implementation of the classic boardgame. If you like this kind of game, do also check out Lux Delux.
The publisher says:
The world is at war, and you are in command of an army fighting for global domination. Organize your forces in a ruthless campaign to crush your enemies and take their territories in this fast paced game of strategy, negotiation, and luck. See the classic game come to life with more ways to play, advanced map options, superior graphics, and animated battles. It's up to you to deploy your troops, attack your enemies, and even betray your allies, in an aggressive effort to take over the world!