Emblem of the Romulan Star Empire.

The Tomed Incident was a horrific act of terrorism in early February 2311 that cost many thousands of Federation lives. History records that Romulan Admiral Aventeer Vokar, an extremist in the Romulan Imperial Fleet, ordered his ship, the Tomed, abandoned by all but six of the crew. From there, Vokar and his cohorts set course for the Foxtrot Sector in Federation space. Upon reaching their destination, the ship impacted with an asteroid base, and its quantum singularity drive caused a massive explosion due to the fact that singularity containment was lost whilst the warp field was still active, disrupting space-time throughout the sector and wiping out dozens of asteroid bases and at least one starship, the USS Agamemnon. Thousands of lives were lost, with only the USS Enterprise-B escaping destruction. Vokar apparently commited this heinous act in the hopes of prompting a war between the Federation and the Romulan Star Empire due to his long-standing Romulan chauvinism and imperialistic stances. The Tomed Incident nearly accomplished Vokar's goal before the Klingon Empire, previously neutral, sided with Federation forces, thus prompting the Imperial Fleet to retreat from the border.

Two months after the Tomed Incident, the Romulan Star Empire forced the Federation to sign a revised Treaty of Algeron that banned the development of cloaking technology within the UFP in return for the Star Empire's agreement to withdraw behind its borders and to recall all of its diplomatic missions and citizens, effectively isolating them on the astropolitical scene. (TLE: Serpents Among the Ruins).

Serpents Among the Ruins

In reality, Vokar did not cause the Tomed Incident, and only the lives of the six Romulans were lost. Capt. John Harriman and Starfleet Intelligence operatives Elias Vaughn and Drysi Gravenor of the UFP stowed aboard the Tomed and tricked most of the crew into abandoning ship. From there, they hijacked the Tomed, ramming it into the Foxtrot Sector asteroid base -- a base that had, like the Agamemnon and the other asteroid bases, been previously emptied of all crew but equipped with special devices designed to emit artificial lifesigns. The deaths of thousands of Starfleet officers from across the Federation had, in the previous year, been covered up, and those officers were then recorded has having all transferred to the Foxtrot bases and to the Agamemnon, completing the fabrication. The Starfleet operatives escaped to the Enterprise, but Vokar and his crew were killed.
As of stardate 41986.0, (26th December 2364) Starfleet and/or the Federation had no contact with the Romulans for 53 years, 7 months, and 8 days (TNG episode 'The Neutral Zone'). This means they last communicate with the Romulan government on the 18th May 2311.
This contact could be when the Treaty of Algeron is signed, or during the month after the signing to allow for diplomats, trade representatives, and other Romulan citizens to be recalled from Federation and Klingon space.
Assuming the latter, the Treaty of Algeron is signed and ratified around the first week of April 2311. This places the Tomed Incident around the first week of February 2311.
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