Originally Posted by
Shellhead
Taking the title literally, we should start with the events of (Uncanny) X-Men #1, circa 1963. In our new scenario IIRC, Arthur is holding the world hostage with nuclear weapons or some-such at a poorly guarded military base in Australia. Something like that. The X-Men show up and get thumped hard by Aquaman. Angel can't do anything that would harm Aquaman. Same goes for Beast and Marvel Girl. Iceman can throw some snow at him, but that's not going to do much good, and snow won't be much protection when Aquaman punches him in the face. Only Cyclops is a threat, but his eyebeams seemed less powerful then, so I think Aquaman can wade through the beam to get at Cyke.
So Aquaman is left in charge of the base, and could even hold it successfully if we grant him his usual Atlantean mililtary. Er, except that Namor rules Atlantis at this time. Or would there be two Atlantises? Anyway, after the X-Men fail, the Avengers might try to save the day, seeing as how they were already Earth's Mightiest Heroes, with a current line-up at that time that included: Thor, Hulk, Iron Man, Wasp, and Ant-Man. Except the Avengers don't have Quinjets yet, so Thor travels to Australia by himself and smites Aquaman without any help. After that, I see Aquaman as being more of one of the under-powered Thor rogues of the day, though nowhere near as weak as Mr. Hyde and Cobra. Aquaman wouldn't have any particular mutant agenda, for or against, so he would rarely if ever face the X-Men again.