War only happens if two agents don’t have common knowledge about who would win (otherwise they’d agree to skip the costs of war).
But that assumes strong ability to enforcement agreements (which humans typically don’t have). For example suppose it’s common knowledge that if countries A and B went to war, A would conquer B with probability .9 and it would cost each side $1 trillion. If they could enforce agreements, then they could agree to roll a 10-sided die in place of the war and save $1 trillion each, but if they couldn’t, then A would go to war with B anyway if it lost the roll, so now B has a .99 probability of being taken over. Alternatively maybe B agrees to be taken over by A with certainty but get some compensation to cover the .1 chance that it doesn’t lose the war. But after taking over B, A could just expropriate all of B’s property including the compensation that it paid.
But that assumes strong ability to enforcement agreements (which humans typically don’t have). For example suppose it’s common knowledge that if countries A and B went to war, A would conquer B with probability .9 and it would cost each side $1 trillion. If they could enforce agreements, then they could agree to roll a 10-sided die in place of the war and save $1 trillion each, but if they couldn’t, then A would go to war with B anyway if it lost the roll, so now B has a .99 probability of being taken over. Alternatively maybe B agrees to be taken over by A with certainty but get some compensation to cover the .1 chance that it doesn’t lose the war. But after taking over B, A could just expropriate all of B’s property including the compensation that it paid.