Why did Frodo feel he had to leave Middle Earth at the of ROTK?

[Zigûr]

Do we have a precise timeframe on when Morgoth started actively trying to turn matter to his will, leeching his essence into the world, as it were? It was clearly after the destruction of the Lamps, because otherwise Aman would presumably have been tainted. Melkor was weakened by the time he was captured in the Battle of the Powers, but that was seemingly, mostly, from putting his essence into his agents, particularly his Maiar servants and his experiments. Did he only start actually imbuing Arda with his spirit after returning to Middle-earth with the Silmarils, or had he already been doing it beforehand? Morgoth's Ring observes that "It is quite possible, of course, that certain 'elements' or conditions of matter had attracted Morgoth's special attention (mainly, unless in the remote past, for reasons of his own plans)." Perhaps this project was begun before Utumno was destroyed but only reached full operation when he was enthroned in Angband.