Much like the game "Risk" you have to take over the world by wisely attacking or defending your squares. Its all based on the roll of the dice. Click your own square to select an attacker, then the enemies to attack it. The more squares you have at the end of a turn, the more bonus dice you get.
I really like risk games, I'm not sure why it chooses to re-enforce your back line though. Oh well, worked out first try took around 10 minutes with everybody playing.