The "umbrella" that protected Canada through most of its history was the British Empire. The USA basically took over the umbrella job after World War II, because the British had passed their apogee of power at that point and the USA was playing World Cop. Canada is essentially safe from direct attack of any great consequence by pretty well anyone BUT the USA, barring nuclear war...but that's an accident of geography. Mexico and Canada have both had the dubious pleasure of being located right next to the USA. Canada was invaded once in the War of 1812, but succeeded in holding its own. The British negotiated away the Pacific Northwest peacefully to the USA at a later date (Washington and Oregon area). Mexico was not so lucky. In a couple of wars they lost California, Arizona, New Mexico, and Texas...all of which were gobbled up by the USA. My, my. Take my word for this: it is dangerous living next to the USA. The Mexicans have never been in any doubt about that. Neither are the Cubans. Neither are we.

The American invasion now, though, is primarily an economic one. They save the military invasions for places a bit farther afield...places with swarthy leaders with facial hair! (gotta be evil to look like that, right?)