How about some kind of fame or prestige point system for actions that are related to wars, like successfully taking over a region, participating in a major battle, commanding a unit with a high CS. It's human nature that players will usually do things that increase their fame and prestige, so create more fame/prestige incentives in war and war-related activities. Conversely, maybe you lose prestige when a region is taken from you. This would encourage players to vigorously defend their regions from being taken over, and would also discourage collusion.
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