Easy, Columbus doesn't get back - his ships all sink on the coral reef off OTL Turks and Caicos forcing him to make land fall. Europe never hears from him again but perhaps a few survivors kick off the spread of diseases into the Western Hemisphere. Over the next 30-40 years native cultures are decimated, but it might give the larger ones a chance to reorganize in time for Europeans to encounter more significant resistance...say, 50-100 later than OTL...