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 Defector"; ENT: "These Are the Voyages...")
The Tomed Incident was the primary focus of the Pocket TOS Lost Era novel, Serpents Among the Ruins. In this version of the story, the Tomed Incident was a staged terrorist attack, where the flagship of the Romulan fleet, the Ivarix-class (β) warbird named Tomed (β), crashed into a Federation base at high warp. When the quantum singularity confinement of the Tomed failed, the massive explosion wiped out thirteen Starfleet outposts, as well as the USS Agamemnon. While the official casualties numbered in the thousands, in reality the Agamemnon and the bases were all empty, rigged to emit false lifesigns. The thousands of Starfleet personnel listed as killed in the attack were already dead – their deaths in the past year having been covered up – and records fabricated to appear that they had been stationed there. The act of terror was orchestrated by Starfleet Intelligence to diffuse the rising tensions between the two powers, forcing the Romulan Empire to stand down when the Klingons sided with the Federation in the wake of the attack.