[Maédhros]

At length Maedhros, having gathered all the strength that he could of Elves and Men and Dwarves, resolved to assault Angband from east and west; and he purposed to march with banners displayed in open force over Anfauglith. But when he had drawn forth, as he hoped, the armies of Morgoth in answer, then Fingon should issue forth from the passes of Hithlum; and thus they thought to take the might of Morgoth as between anvil and hammer, and break it to pieces. And the signal for this was to be the firing of a great beacon in Dorthonion.

Now Morgoth, who knew much of what was done and designed by his enemies, chose his hour, and trusting in his treacherous servants to hold back Maedhros and prevent the union of his foes he sent a force seeming great (and yet but part of all that he had made ready) towards Hithlum; and they were clad all in dun raiment and showed no naked steel, and thus were already far over the sands of Anfauglith before their approach was seen.

I wonder why was it so important to Morgoth to prevent the joining of Forces between Maedhros and Fingon.

, the trumpets of Maedhros were heard at last coming up from the east, and the banners of the sons of Fëanor assailed the enemy in the rear. Some have said that even then the Eldar might have won the day, had all their hosts proved faithful; for the Orcs wavered, and their onslaught was stayed, and already some were turning to flight. But even as the vanguard of Maedhros came upon the Orcs, Morgoth loosed his last strength, and Angband was emptied. There came wolves, and wolfriders, and there came Balrogs, and dragons, and Glaurung father of dragons. The strength and terror of the Great Worm were now great indeed, and Elves and Men withered before him; and he came between the hosts of Maedhros and Fingon and swept them apart.

I have always wondered why didn't Fingon called for the Help of Orodreth.
My suposition that the elves would have won the battle is that some tought that if the host had remain faithful they might have won the day.
The 2 changes that I would assume would make the Nirnaeth Arnoediad a victory for the elves would be the coming of Orodreth and for the treachery of Ulfang to not have occurred. But I suppose that was the Doom of the Noldor.
Yet i accept the wise opinion of Kuruharan.