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"That treaty is the biggest mistake we ever made; it's kept us from exploiting a vital area of defense!"
"That treaty has kept us in peace for 60 years! And as a Starfleet Officer, you're supposed to uphold it.
Rear Admiral Erik Pressman & Captain Jean-Luc Picard, 2370[src]

The Treaty of Algeron was a diplomatic peace treaty signed between the United Federation of Planets and the Romulan Star Empire.


John Harriman, architect of the Tomed Incident.

The treaty was signed in 2311, after the Tomed Incident and approximately 160 years after the Earth-Romulan War. (TNG episode: "The Pegasus")

Representing the Federation was Ambassador Paulo Endara, a Human male. Representing the Romulan Star Empire was Ambassador Gell Kamemor, a Romulan female. Representing the Klingon Empire was Ambassador Kage, a Klingon male. The Treaty reaffirmed the then-existing borders between the Romulan Star Empire and its two neighbors, the Klingon Empire and the United Federation of Planets, as well as the then-existing neutral zones with both powers. In addition, the Treaty banned any development of cloaking technology by the Federation in return for a Romulan withdrawal from the planet Koltaari. (Unknown to the Star Empire, Captain John Harriman had staged an unsuccessful Federation experiment, purported to combine transwarp drive with cloaking technology, to fan Romulan fears that the Federation might seek to develop cloaking technology. As a result, the Federation agreed to give up the pursuit of a technology it never intended to pursue in the first place, cloaking technology, and convinced the Romulans to withdraw from a planet in disputed space without having to engage in military action.) (Novel: The Lost Years: Serpents Among the Ruins)

As of stardate 41986.0, Starfleet and/or the Federation had no contact with the Romulans for 53 years, 7 months, and 8 days, according to TNG episode: "The Neutral Zone". That stardate correlates to late in the year 2364, which means the last communication with the Romulan government would have been in the first half of the year 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.

The treaty reinforced the Romulan Neutral Zone and stated the grounds that would result in the violation by either side which could be considered an act of war. One of the stipulations of the treaty was an express prohibition of the development of cloaking technology by Starfleet. (TNG episodes: "The Defector", "The Pegasus")

The computer game Echoes from the Past states that the Treaty of Algeron created the Neutral Zone and implies this was at the end of the Earth-Romulan war. This interpretation is followed by the DC Comics Convergence story where General Tellius was involved in the Treaty two years after the end of the Earth-Romulan War.

A secret amendment was made to the treaty in 2371 which allowed limited use of cloaking devices by the Federation as long as it was supervised by a Romulan military officer. This allowed the USS Defiant to be loaned a cloaking device for its operations against the Dominion. However, the amendment stated that only a single Defiant-class vessel was allowed the use of the cloaking device and only in the Gamma Quadrant (DS9 episode & novelization: The Search)


Erik Pressman in 2370.

In the year 2358, Admiral Erik Pressman attempted to circumvent the Treaty of Algeron by attempting to create an Interphase cloaking device which was secretly developed by Starfleet Intelligence which was tested on the USS Pegasus. (TNG episode: "The Pegasus")

In 2371, the USS Defiant was deployed at Deep Space 9 for operations against the Dominion and the use of the cloaking device was overseen by Subcommander T'Rul of the Romulan Star Empire. (DS9 episode & novelization: The Search)

On stardate 72487.91, the USS Kelso was lost with all hands. It was revealed that they were testing cloaking technologies, infuriating the remnants of the Romulan Star Empire as well as the Klingon Empire. President Aennik Okeg convinced the other sides that he banned research and construction of Federation-based cloaking technologies. It was stated that the Treaty of Algeron was no longer valid with the Romulan Star Empire as it once was gone. (ST website: The Path to 2409)

Captain William T. Riker.

In 2399, Captain William T. Riker of the USS Zheng He stated that Coppelius, also know as Ghulion IV was under the Federation's protection despite Romulan Zhat Vash General Oh claims in accordance to the treaty. (PIC episode: "Et in Arcadia Ego")

Starfleet launched the Avenger-class battlecruiser in the year 2406, and the USS Avenger was equipped with a cloaking device. Among Starfleet Command, the use of this stealth technology remained controversial but members of Starfleet had argued successfully that the Treaty of Algeron had expired with the fall of the Romulan Star Empire. Those officers were keen to eliminate the tactical advantage Klingon cloaking posed. (STO Collection Issue 11: "USS Avenger")

The Galaxy X-class dreadnought attacks after decloaking.

The Galaxy X-class dreadnought cruiser, launched in 2399,[1] was equipped with a cloaking device by the year 2409. (STO - Romulan Mystery mission: "Preemptive Strike")

By 2410, the Treaty of Algeron was rendered obsolete due to the fact that the Romulan Star Empire had been dissolved due to in-fighting with Empress Sela's disappearance[2] in 2409, leading to the once-mighty Empire being reduced to a handful of hold out colonies outside the Romulan Republic. Without the Treaty, Starfleet was able to expand usage of cloaking technology. (STO - Cloaked Intentions mission: "Darkness before the Light")

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