[Inderjit Sanghera]

The Valar only sent out a Valinorean army to attack Morgoth upon the plea of Earendil. The Noldor, Sindar and Edain would have been overcome, were it not for the intercession of the Valar; and for the Valar's part, the entire thing was far from some grand plan with which to finally bring down Morgoth, as you seem to suggest

The Valar acted at exactly the right-time, they knew of Earendil, and those of them who had a great enough understanding of the Music would know that with Morgoth’s fall would come the Dominion of Men.

When he that is called Earendil setteth forth upon the shores of Aman, ye shall remember my words. In that hour ye will not say that the Statute had born fruit only in death; and the griefs that shall come ye shall weigh in the balance, and they shall not seem to heavy compared with the rising of the light when Valinor growth dim

Statute of Finwe and Miriel

Then, it is said, he stood forlorn looking out to sea, and it was night, but far away he could see a glimmer of light upon Eressëa ere it vanished into the West. Then he cried aloud: 'I will follow that light, alone if none will come with me, for the ship that I have been building is now almost ready.' But even as he said this he received in his heart a message, which he knew to come from the Valar, though in his mind it was remembered as a voice speaking in his own tongue. And the voice warned him not to attempt this peril; for his strength and skill would not be able to build any ship able to dare the winds and waves of the Great Sea for many long years yet. 'Abide now that time, for when it comes then will your work be of utmost worth, and it will be remembered in song for many ages after.' 'I obey,' Círdan answered, and then it seemed to him that he saw (in a vision maybe) a shape like a white boat, shining above him, that sailed west through the air, and as it dwindled in the distance it looked like a star of so great a brilliance that it cast a shadow of Círdan upon the strand where he stood.
As we now perceive, this was a foretelling of the ship# which after apprenticeship to Círdan, and ever with his advice and help, Eärendil built, and in which at last he reached the shores of Valinor. From that night onwards Círdan received a foresight touching all matters of importance, beyond the measure of all other Elves upon Middle-earth.

Problem of Ros; HoME 12

It was evident that Earendil was a 'special' child, in
Laws and Customs of the Eldar (HoME 10) we learn that mother-names are held to be 'prophetic', and the Shibboleth of Feanor says of Idril's naming of Earendil;

More purely prophetic was the name Ardamírë 'Jewel of the World'; for Itarildë could not foresee in her waking mind the strange fate that brought at last the Silmaril into the possession of Eärendil, and enabled his ship to pass through all the shadows and perils by which Aman was at that time defended from any approach from Middle-earth.

'Hail Earendil of mariners most renowned, the looked for that cometh at unawares, the longed for that cometh beyond hope!'

Of the Voyage of Earendil; Published Silmarillion

Evidently the date of the coming of Earendil was not specified, but the Valar knew he would come, to the salvation of Men and Elves when they were at their lowest ebb.

As I have said the Valar timed their attack with the utmost precision, but of course it was Eru who caused all the things to be.

The last intervention with physical force by the Valar, ending in the breaking of Thangorodrim, may then be viewed as not in fact reluctant or even unduly delayed, but timed with precision.
The intervention came before the annihilation of the Eldar and the Edain. Morgoth though locally triumphant had neglected most of Middle-earth during the war; and by it he had in fact been weakened: in power and prestige (he had lost and failed to recover one of the Silmarils), and above all in mind. He had become absorbed in 'kingship', and though a tyrant of ogre-size and monstrous power, this was a vast fall even from his former wickedness of hate, and his terrible nihilism. He had fallen to like being a tyrant-king with conquered slaves, and vast obedient armies.

Myths Transformed; HoME 10

As Sam says (or thinks, as the case was);

For like a shaft , clear and cold ,the thought peirced him in the end that the shadow was only a small and passing thing: there was light and beauty forever beyond it's reach

The Land of Shadow; LoTR

And Gandalf;

But all worthy things that are in peril as the world now stands, those are my care. And for my part, I shall not wholly fail of my task, though Gondor should persish, if anything passes through this night that can still grow fair or can bear fruit and flower again in days to come. For I am also a a Steward. Did you not know?

Minas Trirth; LoTR

So the shadow would never, ever overcome, even if there is little hope left (as there often was) and of course when Gandalf was speaking of him being a steward, he was being a steward of the Valar and therefore representing their thoughts.

With the final defeat of the Elves and Edain where would hope after arise? M-E and maybe all Arda would be crushed.

On Melkors dispersion of powers within Arda itself, we hear much about this in
Myths TransformedArda would forever be tainted with his shadow, as Namo says;

Mandos said: 'Nonetheless they are descendants of Men, who rejected the One himself. That is an evil seed that may grow again. For even if we under Eru have the power to return to Middle-earth and cast out Morgoth from the Kingdom of Arda, we cannot destroy all the evil that he has sown, nor seek out all his servants - unless we ravaged the whole of the Kingdom and made an end of all life therein; and that we may not do.'

Problem or Ros; HoME 12

Melkor's taint would always live on in Arda and never cease to exist whilst Arda lasts.

On the Dagorath, or to be more precise Melkor coming back and what he was doing when he was cast into Ea we hear:

With the final defeat of the Elves and Edain where would hope after arise? M-E and maybe all Arda would be crushed. The dark spirit of Melkor's 'remainder' might be expected, therefore, eventually and after long ages to increase again, even (as some held) to draw back into itself some of its formerly dissipated power. It would do this (even if Sauron could not) because of its relative greatness. It did not repent, or turn finally way from its obsession, but retained still relics of wisdom, so that it could still seek its object indirectly, and not merely blindly. It would rest, seek to heal itself, distract itself by other thoughts and desires and devices - but all simply to recover enough strength to return to the attack on the Valar, and to its old obsession. As it grew again it would become, as it were, a dark shadow, brooding on the confines of Arda, and yearning towards it.

Myths Transformed

That should mean that he was put outside Time and Space, outside Ea altogether; but if that were so this would imply a direct intervention of Eru (with or without supplication of the Valar). It may however refer inaccurately to the extrusion or flight of his spirit from Arda.
In any case, in seeking to absorb or rather to infiltrate himself throughout 'matter', what was then left of him was no longer powerful enough to reclothe itself. (It would now remain fixed in the desire to do so: there was no 'repentance' or possibility of it: Melkor had abandoned for ever all 'spiritual' ambitions, and existed almost solely as a desire to possess and dominate matter, and Arda in particular.) At least it could not yet reclothe itself. We need not suppose that Manwe was deluded into supposing that this had been a war to end war

Myths Transformed

On the actual Dagor Dagoraththe 'Last Battle' we have very few descriptions. There is one in The History of Eriol or Elfwine(BoLT 2) and in the Sketch of the Mythology (HoME 4) and also in the Latter Quenta Silmarillion()HoME 5) in which Namo supposedly, prophecises the events or some of the events that will take place in the Dagor Dagorath. But there is a flaw in this. One in the we learn the 'Valaquenta' that Namo never revealed what would happen in the end of days and in Myths Transformedwe learn that the Silmarillion is in fact of Numenorean origin. The truth is we will know little about the Dagorath because we and the peoples of M-E know so little about it We hear of the Elvish theory that Arda's end will come through the evil nature of Men but the good nature will redeem it. Athrabeth Finrod ah Andreth) (Like a bigger case of the Nirnaeth Arnoediad, maybe.)

We only ever get snatches of info. on the end of Arda, which was inevitable. Arda was not finite. We get a refernce to the 'dark lord' taking over the world in the Barrow-Wights song, who this 'dark lord' was we don't know, Sauron claimed he was indeed Morgoth returned in the T.A, and I doubt whether many of his servants (As the wight was) would have not belived this.

When he found how greatly his knowledge was admired by all other rational creatures and how easy it was to influence them, his pride became boundless. By the end of the Second Age he assumed the position of Morgoth's representative. By the end of the Third Age (though actually much weaker than before) he claimed to be Morgoth returned.

Letter #183; Letters of Tolkien

We also hear that Sauron was 'just a emmisary'of things to come and in Aragorn's cornonation speech,in Quenya, he refers to the worlds end 'Ambat-metta' but this was a echo of Elendil's words upon entering M-E.