Another scenario where a plan like this might have come into use is if Britain had fallen to the Nazis in WWII. Canada being a part of the Commonwealth (without military support from Britain) might have been invaded and used as a staging ground for a US attack and invasion. The US could have invaded and occupied parts of Canada and Mexico (to prevent a southern invasion that would have been more likely) to keep the Nazi's from getting a strong hold in North America. Thankfully none of this happens and our Canadian brethren continue spread their delicious maple syrup and delightful politeness!