The absence of the United States is not going to turn the clock back to the middle ages. Europe and the world still had other strains of liberal democratic thought, ones more radical than those proposed during the revolution in the 13 Colonies. It's not like they had a monopoly on the ideas after all, contrary to some American myths. Simply put different ideas would emerge, some perhaps more radical and tolerant than those historically. The social pressures would build up eventually as the events of 1848 showed.