Canada

Conventionally Canada wouldn't last more than a week. Unconventionally, the insurgency and resistance would tear America to peices. for the same reason Russia's plan for Ukraine was braindead and unfeasible.

When the cultures and appearance of two peoples who are each others neighbors are virtually indistinguishable, the resistance will tear the occupying apart from within. Ukraine has the benefit of having Europe on its border to flood them with Arms, Canada wouldn't have that luxury, the Government would surrender quite quickly, and be replaced with a Quisling government, However resistance against the occupation, the Americans wouldn't know what to do as they'd spend just as much time accidentally hunting their own people, (and lets be real, if something like this happened, America would de-facto already be in a state of civil war as large population centers revolt against a Fascist autocracy that's attacking its own people and America's historical allies and friends for no other reason than plain old fascism)

Sure, but Canada doesn't have the budget to match the mega trillion dollar army the U.S. has.

However, I doubt they would find enough recruits willing to go kill Canadians. They'd probably conscientiously object, except for a handful of bozos. Not to mention they'd probably get sanctioned by every fucking country around the world.