The US imports most of its oil while exporting the oil it produces. Ooops.

The US imports pretty much everything consumers consume. Ooops

The US would have nobody to sell anything to. Ooops

The US is now in a local conflict that a very large % of citizens does not support. What happens after the next election?

These are good points, but I have to comment that all of these suggest the physical resistance to an annexation would come from domestic actors rather than international.

Let me be clear, there are 1,001 reasons why annexing Canada would be a bad idea. I'm just saying that military impossibility is not one of them.

It is a fun fictional exercise, ala Man in the Highcastle

I think it would be closer to Handmaid's Tale than Highcastle since it's a modern day than a twist WWII ending

fun and interesting either way. I hear netflix potential