"Hmm. I am not sure whether or not they could have known that they would be able to level Angband. My instinct is that they did not, but my information is patchy. I should try to tell it in order. The ongoing conflicts gradually escalated, and eventually the conflicting parties held a conference to try to work out their differences, and Melkor planted false evidence that someone would try to do violence to Curufinwë Fëanáro at the conference, so Curufinwë Fëanáro showed up to it armed and armoured, and heard Nolofinwë Arakáno telling Finwë that two loyal sons yet remained to him, and drew his sword and openly threatened to use it if Nolofinwë Arakáno did not desist, which seems to me a counterproductive impulse. And indeed it got Curufinwë Fëanáro exiled from Tirion. He went away and founded another city and half of Tirion followed him, Finwë included, leaving Nolofinwë Arakáno regent in Tirion, and then I don't know what happened until the point at which Melkor brought an abomination from beyond the Void into Valinor to destroy the Trees. I can't see what could be gained strategically from never talking about that entire interval even to his own loyal subordinates, so I can only conclude that Melkor had unfathomable personal reasons for not discussing it."