I am just going to ask this, in what shape or form does Canada have the military to annex the U.S, on top of that NATO?
Even a country like Finland could invade Canada in a few weeks, there is absolutely no chance that Canada would defeat NATO.
Why would anyone want to Annex the U.S.? It is a giant financial mess. Beside which, If lender nations such as Canada (yes Canada is a lender) and China decided to stop doling out money, or worse, even demand payment for what the U.S. owes, they would bankrupt the nation. The U.S. dollar would be destroyed and dethroned as the world currency. Purchasing power and inflation would destroy the American standard of living. The U.S. would become a second world nation; with all its assets up for purchase for pennies on the dollar. Invasion is unnecessary when you can simply buy another country, control the purse strings and rule it from the comfort of your home country. It's hard to maintain a military when you can't afford oil, uranium (which comes from Canada), labour, etc...
With the current choice of President being between Trump and Hillary, many states might voluntarily secede from the union. If the union of American states falls apart, many of those states might choose to become part of Canada, allowing Canada to expand over most of North America.