I can tell you straight up why the world isn't stepping in to try and stop Ghostwalker. Simply put, they can't, not without glassing all of Denver, and even that might not work. Remember, it took a metric shitton of military gear and experimental biomanaweapons to take down Sirrurg, and the fallout from the biomanaweapon used caused everyone in Bogota to go on a mad killing spree. Now what do you think would happen if that kind of thing was launched over Denver? And where would the fallout go? What if, say, it floated over to Chicago, and woke up all those bugs that are still hibernating in the former CZ?
Without another Great Dragon or a bunch of Immortal Elves, you're not going to take Denver from Ghostwalker.