MindzEye
Wayward Ditch Pig
the question is who would they invade first. in that case, my money is on Mexico.
I also think you err by applying logic to any of this.
I think they start a CIA driven clandestine war against the cartels first. The CIA has a long and proud history of nurturing guerilla & criminal groups and then turning on them when they become too big of a problem or priorities change.
Our sparsely populated, beloved northern tundra though? Full of polite people that speak the right language, own few weapons of war and would struggle to organize a defence before every population centre had American troops controlling them? Easy first choice.