Tomed Incident

From Memory Alpha, the free Star Trek reference

The Tomed Incident was a terrible confrontation between the United Federation of Planets and the Romulan Star Empire in 2311 that cost thousands of lives. The incident led to the signing of the Treaty of Algeron, which banned Federation research into or use of any cloaking device, and led to the withdrawal of the Romulan government from interstellar affairs until 2364. (TNG: "The Neutral Zone", "The Pegasus")

The Tomed Incident was the primary focus of the Lost Era novel, Serpents Among the Ruins.

The exact length of time of the Romulan isolation has been retconned several times during the production of TNG and later series and movies. In "The Neutral Zone", Data stated it was 53 years, seven months and 18 days since the last official contact between the Federation and the Romulan Star Empire. Others have postulated that the isolation itself has not been retconned, but that contacts with the Romulans known to have occurred between 2311 and 2364 are the result of contacts with non-governmental Romulan agencies, unofficial contacts that were not recorded in the historic record, or agencies gone rogue.