Treaty of Algeron
The Treaty of Algeron was a peace treaty signed between the United Federation of Planets and the Romulan Star Empire in 2311, following the events of the Tomed Incident. The treaty reinforced and re-defined the Romulan Neutral Zone created by the Treaty of Cheron in 2160, and made clear that any violations of the Zone without adequate notification, by either side, would be considered an act of war.
The Treaty of Algeron has shaped the course of Federation relations with the Romulan Empire and dictated Starfleet's use of technology. The use of a cloaking device by a Federation starship is strictly prohibited by this treaty. Admiral Pressman attempted to circumvent this clause in 2358 with the test of an interphase cloaking device developed by Starfleet Intelligence aboard the U.S.S. Pegasus.
A treaty signed between the Federation and the Romulan Empire in 2371 waived the Treaty of Algeron and enabled the U.S.S. Defiant to install and operate a cloaking device. In return, the Romulans would receive all information the Federation obtained on the Defiant's voyages into the Gamma Quadrant, especially any information they found on the Dominion.