Valar assault Utumno (4590 F.A.) - Once Orcs began to cover the land, the Valar knew it was the work of Melkor.  They set out to put a stop to his evil deeds and laid siege to his dark abode at Utumno.  What followed became known as the Battle of the Powers.

 

"Then Manwë said to the Valar: 'This is the counsel of Ilúvatar in my heart: that we should take up again the mastery of Arda, at whatsoever cost, and deliver the Quendi from the shadow of Melkor.'

 

"...and thereafter, the Quendi knew nothing of the Battle of the Powers, save that the Earth shook and groaned beneath them, and the waters were moved, and in the north there were lights as of mighty fires.  Long and grievous was the siege of Utumno, and many battles were fought before its gates...

 

"But at last the gates of Utumno were broken and the halls unroofed, and Melkor took refuge in the uttermost pit.  Then Tulkas stood forth as champion of the Valar and wrestled with him, and cast him upon his face; and he was bound with the chain Angainor that Aulë wrought, and led captive; and the world has peace for a long age.  Nonetheless the Valar did not discover all the mighty vaults and caverns hidden with deceit far under the fortresses of Angband and Utumno.  Many evil things still lingered there, and others were dispersed and fled into the dark and roamed into the waste places of the world, awaiting a more evil hour; and Sauron they did not find." ~The Silmarillion, chap. 3 (Of the coming of the Elves)