Well Hiroshima, Nagasaki, and Chernobyl haven't really spread beyond their borders but retaliation is the big problem. UPDATE President Vladimir Putin claims that the rebels (who were supplied by America) were the ones who spread the nerve gas. Not the Syrian government.
He also said that America is a force that takes over countries under the principle that "if you're not with us your against us". Those are fighting words where I'm from (which I shall not reveal).