The Recapture of north-east Beleriand

[Macalaure]

Formendacil, I didn't say it was a fatal error, I wonder why Tolkien does.

Here's the passage from the Silmarillion:

But Maedhros made trial of his strength too soon, ere his plans were fullwrought; and [...] Morgoth was warned of the uprising of the Eldar and the Elf-friends, and took counsel against them. Many spies and workers of treason he sent forth among them, as he was better able now to do, for the faithless Men of his secret allegiance were yet deep in the secrets of the sons of Feanor.

So Ulfang was already in league with Morgoth. Either he had not told Morgoth until then, or Morgoth didn't listen/believe him.

Which makes me think:
If Nargothrond and Doriath were in the Union, would Maedhros have asked the Easterlings to join at all?