Nuclear disarmament wasn't the only reason Russia was able to "annex" Crimea. A massive media blitz convinced many of the civilian population that they were being overrun by revolutionists, thugs, immigrants, etc and that only Russia could save them. I believe it's highly likely Russia would have succeeded even if Ukraine had all of their nuclear weapons in hand. Firing even one at Russia would have resulted in the rest of Ukraine being wiped off the map.
