Mutual consensus among multiple - say, at least 5 - nations chosen for maximum mutual hostility, that leads to proportional reduction of military spending by sum of 10% of world's total military spending, done honestly and in a way that doesn't alter the balance of power.
I.e. the kind of coordination game theory decrees as effectively impossible. Were such event to happen, whatever mechanism drove it could be used to reduce emissions and implement effective climate change mitigations pretty much on the spot. And if it generalizes as solution to coordination problems, it would literally solve all major issues plaguing humanity to date.
