The Valar 'fade' and become more impotent, precisely in   proportion as the shape and constitution of things becomes   more defined and settled.  The longer the Past, the more   nearly defined the Future, and the less room for important   change (untrammelled action, on a physical plane, that is   not destructive in purpose).  The Past, once 'achieved', has   become part of the 'Music in being.'  Only Eru may or can   later the 'Music'.  The last major effort, of this demiurgic   kind, made by the Valar was the lifting up of the range of   the Pelori to a great height.  It is possible to view this   as, if not actually a bad action, at least a mistaken one.   Ulmo disapproved of it.  It had one good, and legitimate,   object: the preservation incorrupt of at least a part of Arda.   But it seemed to have a selfish or neglectful (or despairing)   motive also; for the effort to preserve the Elves incorrupt   there had proved a failure if they were to be left free: many   had refused to come to the Blessed Realm, many had revolted   and left it.  Whereas, with regard to Men, Manwe and all the   Valar knew quite well that they could not come to Aman at all;   and the longevity (co-extensive with the life of Arda) of   Valar and Eldar was expressly not permitted to Men.  Thus the   'Hiding of Valinor' came near to countering Morgoth's   possessiveness by a rival possessiveness, setting up a private   domain of light and bliss against one of darkness and   domination: a palace and a pleasuance (well-fenced) against a   fortress and a dungeon. 

   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. 

   Nontheless the breaking of Thangorodrim and the extrusion of   Melkor was the end of 'Morgoth' as such, and for that age   (and many ages after).  It was thus, also, in a sense the end   of Manwe's prime function and task as Elder King, until the   End.  He had been the Adversary of the Enemy. 

   It is very reasonable to suppose that Manwe knew that before   long (as he saw 'time') the Dominion of Men must begin, and the   making of history would then be committed to them: for their   struggle with Evil special arrangements had been made!  Manwe   knew of Sauron, of course.  He had commanded Sauron to come   before him for judgement, but had left room for repentence and   ultimnate rehabilitation.  Sauron had refused and had fled into   hiding.  Sauron, however, was a problem than Men had to deal   with finally: the first of the many concentrations of Evil   into definite power-points that they would have to combat, as   it was also the last of those in 'mythological' personalized   (but non-human) form. 

   The last effort of this sort made by the Valar was the raising   up of the Pelori -- but this was not a good act: it came near   to countering Morgoth in his own way -- apart from the element   of selfishness in its object of preserving Aman as a blissful   region to live in. 

   The Valar were like architects working with a plan 'passed' by   the Government.  They became less and less important   (structurally) as the plan was more and more nearly achieved.   Even in the First Age we see them after uncounted ages of work   near then end of their time of work -- not wisdom or counsel.   (The wiser they became the less power they had to do   anything -- save by counsel.) 

   Similarly the Elves faded, having introduced 'art and science'.   Men will also 'fade', if it proves to be the plan that   things shall still go on, when they have completed their   function.  But even the Elves had the notion that this would   not be so: that the end of Men would somehow be bound up with   the end of history, or as they called it 'Arda Marred'   (Arda Sahta), and the achievement of 'Arda Healed'   (Arda Envinyanta).  (They do not seem to have been clear   or precise -- how should they be! -- whether Arda   Envinyanta was a permanent state of achievement, which   could therefore only be enjoyed 'outside Time', as it were:   surveying the Tale as an englobed whole; or a state of unamrred   bliss within Time and in a 'place' that was in some sense   a lineal and historical descent of our world or 'Arda Marred'.   They seem often to have meant both.  'Arda Unmarred' did not   actually exist, but remained in thought -- Arda without   Melkor, or rather without the effects of his becoming evil;   but is the source from which all ideas of order and perfection   are derived.  'Arda Healed' is thus both the completion of the   'Tale of Arda' which has taken up all the deeds of Melkor,   but must according to the promise of Iluvatar be seen as good;   and also a state of redress and bliss beyond the 'circles of   the world'.)