The Treaty of Algeron was a peace treaty signed between the United Federation of Planets and the Romulan Star Empire in 2311 on the planet Algeron IV, following the events of the Tomed Incident. The Treaty of Algeron was signed approximately 160 years after the conclusion of the Earth-Romulan War. (TNG: "The Pegasus")
The treaty reinforced and redefined the Romulan Neutral Zone, and made clear that any violations of the Zone without adequate notification, by either side, would be considered an act of war. (TNG: "The Defector"; ENT: "These Are the Voyages...") The treaty also expressly prohibited the development or use of cloaking device technology by the Federation. Admiral Pressman attempted to circumvent this clause in 2358 with the test of an interphase cloaking device developed covertly at least in some part by Starfleet Intelligence aboard the USS Pegasus. (TNG: "The Pegasus")
A secret amendment added to the treaty in 2371 allowed for the limited supervised use of at least one cloaking device on loan from the Romulans. Operation of the cloaking device was to be limited to the Gamma Quadrant. In return, the Romulans would receive all information the Federation obtained on the USS Defiant's voyages into the Gamma Quadrant, most importantly any information concerning the Dominion. As such, the Defiant was the only Federation ship to be permanently equipped with a cloaking device (DS9: "The Search, Part I", "Visionary"). However, Captain Benjamin Sisko and his crew violated this amendment on several occasions by using the cloaking device in the Alpha Quadrant. (DS9: "The Way of the Warrior" et al)
- While not mentioned on screen, its possible Section 31 was the part of Starfleet Intelligence responsible for the mandate of, and subsequent cover up for the previously mentioned Phase Cloak Device.
- In Q's anti-time future, it was established that the Klingons had conquered the Romulan Star Empire. The treaty was clearly negated, as the Federation possessed and openly used cloaking technology. (TNG: "All Good Things...")