I think the Brits will stand with us. After all, they're family. Besides, they kind of owe us. Switzerland and Sweden will hem and haw about neutrality and the UN and all that crap. France will try to go it alone and go down in flames very quickly. Germany will be paralyzed by neurotic guilt. Japan will shrug and say it's not their problem. India will go it alone while making snide comments about Western imperialism and the excesses of the capitalist system. They'll have a nuclear exchange with Pakistan that will solve their overpopulation problem in short order. Australia will get staggering drunk and sing "Waltzing Matilda" all night long. Israel will fight like a cornered rotweiller. The Irish are bound to join the fight, but it's anyone's guess on whose side. Russia will drop the last pretentions to democracy for the sake of national security.
Canada will do its part in a modest way. But Bruce Cockburn will steal a rocket launcher and defect to the enemy.
Microsoft will declare itself a sovereign nation, and then succumb to a massive script kiddie attack.