Well, if you insist :-). Some of this explanation made it into the published version of the Silmarillion, but much more is found in the History of Middle Earth series that contains compilations of Tolkien’s drafts, outlines, essays, and so on. The most important of these for our purposes is Volume X: Morgoth’s Ring.
Essentially, Tolkien saw Melkor and Morgoth as distinct entities. Melkor was the mightiest of all created beings, second only to Iluvatar and stronger than any individual Valar (in earlier conceptions) or all of them combined (in later drafts). His power was so great that he alone was able to contend with the Valar and Maiar during the ages before the birth of the Two Trees. His fatal flaw, however, was his lust for dominion over both other individuals and the actual fabric of the world itself. That lust caused him to dissipate the vast majority of his power into base matter and slaves/servants like orcs, dragons, Balrogs, werewolves, vampires, Sauron, and so on. As a result of this dispersal, the hugely reduced being that remained became Morgoth. I’ll quote various passages from Tolkien at length on this topic. These are from his own musings, essays he wrote to guide his more polished fiction:
Sauron was ‘greater’, effectively, in the Second Age than Morgoth was at the end of the First. Why? Because, though he was far smaller by natural stature, he had not yet fallen so low. Eventually, he also squandered his power in the endeavor to gain control of others. But he was not obliged to expend so much of himself. To gain domination over Arda, Morgoth had let most of his being pass into the physical constituents of Earth–hence all things that were born on Earth and lived on it and by it, beasts or plants or incarnate spirits, were liable to be ‘stained’…Sauron, however, inherited the ‘corruption’ of Arda, and only spent his (much more limited) power on the Rings; for it was the creatures of earth, their minds and wills, that he wished to dominate…
For this reason, (Morgoth) had to be fought, mainly by physical force, and enormous material ruin was a probable consequence…Manwë’s task and problem was much more difficult than Gandalf’s. Sauron’s relatively smaller power was concentrated, Morgoth’s vast power was disseminated. The whole of ‘Middle Earth’ was Morgoth’s Ring, though temporarily his attention was mainly upon the North-west…Moreover, the final eradication of Sauron was achievable by the destruction of his Ring. No such eradication of Morgoth was possible, since this required the complete disintegration of the matter of Arda. (Tolkien goes on to describe how certain elements, like gold, contained more of Melkor’s power than others, like silver and water).
This is Tolkien’s version of Original Sin. Everything that lives on Earth contains some of Melkor’s essence and is therefore prone to evil. The next passage discusses the dire personal consequences to Melkor of his diminishment prior to the Battle of the Powers, which took place when the Valar attacked Utumno, Melkor’s first fortress, shortly after the awakening of the Elves (Quendi) .
The war against Utumno was only undertaken by the Valar with reluctance, and without hope of real victory, but rather as a covering action or diversion, to enable them to get the Quendi out of his sphere of influence. But Melkor had already progressed some way towards becoming ‘the Morgoth, a tyrant (or central tyranny and will), + his agents’. Only the total contained the old power of the complete Melkor; so that if ‘the Morgoth’ could be reached or temporarily separated from his agents he was much more nearly controllable and on a power level with the Valar. The Valar find that they can deal with his agents (armies, Balrogs, etc.) piecemeal. So that they come at last to Utumno itself and find that ‘the Morgoth’ has no longer sufficient ‘force’ (in any sense) to shield himself from direct personal contact. Manwë at last faces Melkor again, as he has not done since he entered Arda. Both are amazed: Manwë to perceive the decrease in Melkor as a person; Melkor to perceive this also from his own point of view: he now has less personal force than Manwë.
This dispersal only got worse after the end of his imprisonment. When Melkor empowered Ungoliant to attack the Trees, he lost forever the power to change his shape. When he returned to Middle Earth and created seemingly endless hordes of orcs, trolls, dragons, and whatnot, he became ‘ever more bound to the Earth’:
For now, more than in the days of Utumo ere his pride was humbled, his hatred devoured him, and in the domination of his servants and the inspiring them with lust of evil he spent his spirit.
The apparently futile battles of the War of the Jewels actually had very positive consequences, as the casualties Morgoth’s armies suffered steadily weakened his total power. Thus it was that Fingolfin, one of the mightiest Elven warriors of the First Age, could wound Morgoth eight times in their duel before the gate of Angband. Morgoth was stronger than any Elf, but he was no longer Melkor, who would have been completely unassailable.
