Battle of Cheron

The Battle of Cheron in the history of Star Trek is the victory of pre-Federation Earth forces over the Romulan Star Empire. Little is known now about its circumstances, but Earth's victory indirectly led to the formation of the United Federation of Planets and cut off all contact with the Romulans for just over a century.

Even by 2366, the Battle of Cheron had not been forgotten by the Romulans (from the Star Trek: The Next Generation episode "The Defector").