If by the demon's gods you mean those demon lords who have worshippers, such as Baphomet, Demogorgon, Jubilex, etc then they're only Demipowers. As such, they wouldn't stand a chance against the combined might of even just Faerun's Lesser Powers, let alone the Intermediate and Greater Powers.
As for the other evil gods of Baator and the Abyss, most of them have mortal worshippers, so they'd lose vast amounts of power (or die in some cases) if those worshippers got killed off. Thus even Lolth, Beshaba, Umberlee, Sekolah etc would fight back against such an invasion. Pretty much the only evil deity of Faerun I can think of who might support the demons/devils would be Shar, since her ultimate goal is the end of the world.
BTW the celestials of the Upper Planes are hardly a pushover. Perhaps the fiends would win in the end, but the war would almost certainly last for thousands of years.
Edited by NiGHTMARE, 04 July 2005 - 06:15 AM.