Tboy
Future world ruler
Having a random event do that rather than the actual game seems a bit artificial though. Have you tried Rhye's and Fall of Civilization? Rhye changed defensive pacts to allow up to 4 civs to engage in them at the same time, which leads to big world wars, especially because the civs in the defensive pacts probably have vassals.
Plus they're military alliances more than defensive pacts, i.e. declaring war will drag them in as well, and the pact remains.