That'd be the dumbest thing the US has ever done. The next 50 years would be as Vietnam, with people indistinguishable from Americans, sneaking across the border 8000km long, to explode, and kill, and destroy. A Vietnam that never ends, and is local.
Not to mention, Canada can make nukes if it wants to. It just doesn't. It gave them up, but still has the plans.
I don't think it would be a military invasion, more like an economical one.
Canada relies on US and is isolated from the rest of the world, US can strangle it economically if need be until it folds.
They can also cause divisions inside the country and disrupt it.. we're seeing this already in Alberta.
Canada is vulnerable.