Canada, like the US is a member of NATO or North Atlantic Treaty Organization. It's a defencive alliance that was founded during the cold war to counter-act the spread of communism and to fight the USSR if they ever decided to go to war. The conditions of the alliance is that no two members may wage war against one another and all members of the alliance must declare war upon an attacker of one of its members. So if the US attacks Canada(which has very little oil compared to the US) the US would be breaking the rules of its Alliance and would have war declared on it by Most of Europe, Canada, and any other countries that are part of the alliance. We don't attack because Canada's our ally and we aren't stupid. [Edited on 10.03.2012 5:22 PM PDT]