The UN would act very quickly indeed. Being Canadian, I was quite pleased that our Prime Minister did not join the US in their folly-ridden invasion of Iraq. Whatever the purposes of the President - though I think they were likely ones of conquest rather than liberation - they have yielded very little benefit, and have not served to stabilize the area. Thus I think that, if such an event were to occur, we would need to trust to the UN security council, rather than the rash actions of one rebellious country with an itchy trigger finger. The US, as it has so recently done, would probably attack, but as the UN would at once approve all war measures, I don't think there would be much debate. Now, let's see how the war would occur.
Firstly, we see from the speed of Lavos' assault that very little is left, in a sort of Independence Day style strike. Not everything can be hit, however, and this give the chance of a counterattack.
Firstly, the non-US countries can do very little. They would follow what I outline in the following as first strike options for the US, but do not have the final nuclear power neccessary to finish the deed. So we'd probably see a massive marshalling of Tu-144 Blackjacks - do the Russians even have any left, come to think of it? - and all sorts of strike fighters. The ground forces would be decimated, so any strike would be aerial. Carriers here would probably be the way to go as well. Canada... well, we'd be helpless here. Our helicopters would probably crash before we crossed our border. Sorry, world, we tried. Anyway, NATO would give some measure of support as well with Tornadoes, and all their LGBs. Sea-based cruise missiles would be launched en-masse, and this would severely weaken Lavos.
As far as the US goes, I think their first choice would be a tactical strike through the use of F-15 Strike Eagles and Joint Strike Fighters, likely using the sort of bombs meant to penetrate bunkers. This would probably fail, and they would instead opt for a full-scale conventional bombardment via. any B-52s they have, and their ever-impressive largest bombers, B-1 Lancers. Again, they'd probably be armed with bunker-busting high power laser guided bombs, and perhaps some of those nasty ones that cause implode all air to their centres. Again, though this may weaken him, it would likely fail. Finally, they'd resort to Nuclear Weaponry. No tactical nukes here, they'd probably go straight to DefCon 1, or whatever Nuclear War is, and launch ICBMs. As strong as Lavos is, I honestly don't think even he could survive a 20MT nuclear warhead; the radiation and fallout effects would be neglidgable. The US detonated one in Bikini Atoll in the 50's, and all the area about Lavos would be rendered ruin as it is. Someone mentioned Neutron bombs, and I figure that, also, would be a good choice. That would be the reprisal, and as much as I'd hate to admit it, the world would owe its safety to the militaristic Americans. After all, I don't think anyone else has the strike power to deal with such a threat. I figure the US would be all but destroyed however, a last stroke of glory before their doom; it's likely where Lavos would surface, and so most of the central states would be rendered barren as a field of a nuclear war.