In 2AD the Romans gave out citizenships to everyone who lived in the Roman Empire. Those Greeks who lived within in the Roman Empire still felt the need to hold these athletic games. This spread through the Roman Empire which made the Games international.
In 1896 the Greeks officially started the Olympics again after 18 centuries. They held onto the aspect of them being international games. Now some 120 years after the Games were re-instated there are over 10'000 athletes from 205 countries participating in 300 events.
information: http://en.wikipedia.org/wiki/Olympic_Games
picture: http://www.hdwallpapersinn.com/olympics-rings-hd-wallpapers.html