Overmatched

What possible circumstances would have made Dr. Doom agree to lead the Fantastic Four into battle?

Well, it may have had something to do with this bruiser:

The Over-Mind was a conqueror of the first order, a threat not only to Earth but eventually the universe. When it came time for the Fantastic Four to meet that threat, the Over-Mind drew first blood by enslaving their leader, Reed Richards, to his will, effectively making him his lackey and turning him against the FF.  On the bright side, while Reed was under the Over-Mind's control and battling his teammates, we did get to learn that while the Thing can prevail against some fierce and brutal attacks, he can still be taken down by a different kind of assault:

(Let's hope for Ben's sake that this is never leaked to the Yancy Street Gang.)

The Thing and the Torch nevertheless engaged the Over-Mind in battle--while the Invisible Girl came under attack by Reed, and barely escaped him. (This was well before Sue became a more capable member of the team.  Though to be fair, if you take Reed's head out of his lab and give him a mad-on, he's no slouch in the battle department.)

Distraught, and finding no other allies to help them (apparently all the other Marvel heroes were unavailable, too busy with their own affairs--ridiculous, but necessary story-wise to isolate the FF), Sue, faced with no other choices, appealed to Dr. Doom for assistance. Doom, at first, couldn't be bothered with helping his arch-enemies avoid defeat; but after Sue all but accused him of being a coward, he relented.

Modifying a design of Reed's, he brought a "psionic-refractor" to the battle site--which turned the Over-Mind's own mental power against him. Yet the Over-Mind, being no stranger to battle tactics, deduced that Doom was using the Thing and the Torch as distractions, in order to disguise the true threat of Doom. Consequently, he focused the brunt of his attack on Doom, who then made use of Sue's force field to blunt the force of the attack until the Over-Mind was felled by feedback from the refractor.

And it might have worked, were it not for Reed appearing and once again attacking Sue, leaving Doom defenseless against the Over-Mind's onslaught. The refractor shattered under the barrage--though Doom, proud and defiant, wasn't about to go down without a fight:

Not much of a fight, as it turned out. The Over-Mind's powers were not only huge, but growing--and neither Doom nor the FF were really a match for him. Yet by fighting so fiercely against this foe, the FF forced the Over-Mind to expend his full force (at least to that point) against them--which inadvertently caused another powerful galactic figure, the Stranger, to detect the Over-Mind on Earth. The Stranger--a being finally revealed to have gone through a similar process that created the Over-Mind, only with far greater power--then confronted the Over-Mind, and promptly disposed of this growing threat to the universe.

A very abrupt ending to the battle, but noteworthy in that it had the Over-Mind on the verge of conquering Earth, but for the intervention of one whose origins were like his own. And though it appeared that the Over-Mind had his hands full with just conquering New York, his powers were such that continued use of them had the effect of making him more and more powerful. According to the Watcher, even the Stranger wouldn't have been able to stop him once he'd conquered a few more worlds.

As for Doom, once the battle was over he made it quite clear to the FF that this was a one-time alliance, and that they'd be enemies again at their next meeting. A conclusion perhaps reached, in part, by the notion that if this was a typical FF battle, he was probably better off fighting against them than with them!