Tomed Incident

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

According to one account, the Tomed Incident was a staged terrorist attack, when the Romulan flagship Tomed crashed into a Federation base at high warp. The quantum singularity confinement that powered the Tomed failed, and the explosion destroyed thirteen Starfleet outposts, as well as the USS Agamemnon. With all Federation bases and starship[s] rigged to emit false lifesigns, the listed dead had already been deceased, their deaths and causes of already covered up. This terror attack was said to have been staged by Starfleet Intelligence, in order to defuse the rising tensions between Romulan Government and Starfleet. When the Klingons sided with the Federation, the Romulans stood down.