German-japanese alliance in WW II
it didnt actually benefit either of them, but because of perl harbor, america could come off their semi non interference policy. If america (and canada, and australia and all their allies) hadnt joined the fight, d-day may never have happened,
and the alliance with japan was most likely to distract russia, but japan moved towards the isles and not the mainland, which defeated the third reich twice over by drawing in the USA and not dividing the USSR