Well, think about it. We'd have all the oil we'd need (Mexico and Canada) All the Natural Gas, too. All the cheap labor, all the cheap votes, and all the Cuban cigars we could smoke!..................
There is an easy resolution to this problem.
Simply annex all of Canada, with the exception of Quebec (which would be allowed to declare sovereignity), then admit each of the provinces individually as states.
Then annex all of Mexico, admitting each each of their states on an individual basis as well, as soon as they have achieved a basic competence in English for the majority of their citizens, and the legal code is reformed so it comforms with, or at least eliminates the parts that conflict with, common law in the United States.
Now, stop bothering us about that. We got a global war to fight.