While everyone knows that America is overprotective of Canada, it is far less known that Canada is just as, if not more so, protective of America.
Japan found this out the hard way in WWII*.
Seriously, if you do anything to hurt America, you may want to put your affairs in order, because Canada will be hitting you when you least expect it. And may God have mercy on your soul if you break America's heart.
(*After the attack on Pearl Harbor, Canada was the first one to declare war.)