The collapse of the Roman Empire
Quick history lesson for all of you out there, the Roman Empire was a large empire that conquered most of Europe, parts of Africa, and even Asia during its height. After hundreds of years, the empire was eventually taken down when a group of barbarians sacked the capital in Rome.
I bet they would have never believed that their culture, history, and legacy would carry on so far into modern times that we still use a lot of it today. I bet they also would have never believed that someone could not understand that roman numerals are letters. So much for the empire.