IME, Canadians and Americans have no trouble talking. Consequently, if the US invaded Canada, it would be a civil war, yes? And, since the British and Americans both speak English, if armed forces were sent from the England into the US, that would also be civil war. Nearly every person in Europe speaks English as well as their native language; given that they can easily understand each other, any war between nation-states should be classified as a civil war.
It's an interesting definition of civil war, that's for sure.