Asgardian-Olympian War
According to the history, hundreds or thousands of years earlier, Asgard and Olympius went to war the first time they encountered each other. Very young Thor and Hercules met on the battlefield, but it didn't take long for Odin and Zeus to make peace and an alliance, which holds to this day. Apparently, it was so together they could fight the return of the Celestials, leading to the creation of the Destroyer armour etc.
I don't know how much this was covered on the comics page, but it would ripe for a big mini-series now, I say! If DC can do the Rann-Thanagar War, then this needs revisiting.