A deadly confrontation between the Federation and the Romulans in 2311, after which the Romulan Star Empire withdrew from contact for more than fifty years.
The Tomed Incident was a violent clash between the Federation and the Romulan Star Empire in 2311 that cost thousands of Federation lives. Few details have been revealed on screen, but it marked a turning point in relations between the two powers.
In its aftermath the Treaty of Algeron reaffirmed the Romulan Neutral Zone and included a provision barring the Federation from developing cloaking technology, a clause that drives the plot of the episode The Pegasus. The Romulans then withdrew behind their borders and avoided contact with the Federation for over half a century, leaving Starfleet uncertain of their intentions.
The incident is referred to in The Neutral Zone, the first season finale of The Next Generation, set in 2364. When Romulan outposts along the border begin disappearing, a Romulan warbird appears and its commander tells Jean-Luc Picard that the empire is ending its long isolation. The Tomed Incident therefore explains why the Romulans were absent for so long and sets the stage for their return as a major antagonist of the series.