[In Response to: The devil already lost one battle against God, and it sounds like it wasn't even close.]
The devil was outnumbered 2 to 1 according to the most common narrative I've heard. Defeat makes sense even when you ignored the fact that the winning side included an omnipotent enemy.
Anyway, this is where the devil in any narrative I'd make about this points out that: a) They've been breeding an army in the intervening years while God hasn't made a new angel since creation. Now Hell out numbers Heaven. b) Back then the world wasn't fallen yet, and that fact was what gave God the upper hand. Now that the world is fallen the balance of power has shifted. c) All of the souls now damned to Hell have been tapped as an energy source that will be used to tip the balance of power in favor of Hell. d) Many of the loyalist angels have had second thoughts since then, having seen what God did to their rebel friends, Hell has allies inside of Heaven who will, at the opportune moment, attack. e) A lot of the angels who stayed loyal did so not because they liked God, but because they liked humans, now that God is exterminating the humans they have no reason to remain on Heaven's side, meanwhile the devil can say (honestly or not, depending on the story) that he's reconsidered his stance and really doesn't wish humanity harm. f) Christopher Walken The Archangel Gabriel has started a second war in Heaven, meaning that the army of Heaven has been killing itself off for a while, now is an ideal time to strike. g) Back then they didn't have [insert Hell's only hope here]. h) Fight? Why the Hell would we fight? We're trying to escape. (If evil: We're going to blow up the Earth as a distraction and then sneak away in the kerfuffle.) or i) Anything that would make this not a foregone conclusion.