Here is the scenario.
Spain does not explore Mexico and Central America, same a Portugal. This means that the English and French still have their colonies. But they never investigate the southern areas. After the American Revolution the Aztecs investiagte the north and encounter the Americans. They slay all French settlers and build cities on top of their settlements. America, Britan and France stay out of contact. 1813 arrives and the British never start the war of 1812. But the Aztecs launch a suprise attack on the Americans. What will happen?