Trains and highways are required for moving armor and men that far north. The moment movement is noticed, those tracks and roads will become a lot less vehicle friendly. A lot of those northern states would not take too kindly to our neighbor being invaded. Idaho, Montana, and Alaska are the only areas where they would face less issues with movement through the state. Maybe North Dakota and Maine.

It's not a brilliant move for the US to do. It wouldn't work. Its too wide a border.