Morgoth Corrupts MenMorgoth corrupts Men - With the Awakening of Men, Morgoth saw in them the potential to gain a new ally.

 

"But it was said afterwards among the Eldar that when men awoke in Hildórien at the rising of the Sun the spies of Morgoth were watchful, and things were soon brought to him; and this seemed to him so great a matter that secretly under shadow he himself departed from Angband, and went forth into Middle-earth, leaving to Sauron the command of the War...To corrupt or destroy whatsoever arose new and fair was ever the chief desire of Morgoth; and doubtless he had this purpose also in his errand: by fear and lies to make Men the foes of the Eldar...But this design was slow to rippen and never wholly achieved..." ~The Silmarillion, chap. 17 (Of the coming of Men into the West)

 

This explains why some Men are allies of the Elves while others are enemies.