[denethorthefirst]

Melkor can't achieve either of those goals, so in that sense, he can't "win". The most he can hope for is some kind of stalemate/Cold War. But even if he somehow repelled the Host of the Valar: the Valar can just rehouse their fallen elven soldiers ad infinitum can't they? At that point the conflict becomes a war of attrition that Melkor will surely lose. The only question would be if the High-elves would have the stomach for the necessary decades long carnage, but considering that the actual War of Wrath went on for roughly 40 years (!!!) I guess that's not a problem. We also have to remember that the Valar restrained themselves to limit the damage and didn't actually participate in the war; if the initial invasion fails they may reconsider that approach and go all in.