The answer to this is the lesser houses of Men. From Athrabeth Finrod ah Andreth: So already before the Edain met the Eldar, there was grief among Men. Why is this? the answer can be found in The 'Tale of Adanel.' After Men had been deceived away from Eru, he came back and spoke to them one more time: And Men grew afraid, but fearing their Master, still did his bidding. But: Does not this description fit others that we have heard of from the Silmarillion? Brodda the easterling, and Lorgan. Men, who were powerful and proud, and who enslaved other Men. And let us not forget Men like the sons of Ulfang, who at the turning point of the Nirnaeth Arnoediad And so the eastern defences fell to the power of Morgoth. And after this, Hithlum fell, and Doriath was ruined, and last of all fell Gondolin. Men fought against Men, Dwarves, Elves and Men were alienated, against eachother. The only peoples who kept a firm alliance against Morgoth, were Elves and the Men of the Three Houses of the Edain. And from them came Eärendil, and the downfall of Morgoth.