Many times in history, a very skilled leader leads a nation to conquer a much more powerful one. This scenario is unlikely, but not ASB. What if Mexico conquers the Thirteen Colonies? They could have lost the Revolutionary War and are revolting again, or their war was much more bloody than OTL, or the Articles of Confederation remain in place and the nation falls apart. Or a combination of those. If not even a Mexico with, say, Bonaparte on its side could conquer even a divided Articles US that fought a bloodier revolution--what are the effects of a Mexican invasion that occupied about half the country and is then pushed out?