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 on the planet Algeron IV, following the events of the Tomed Incident.
Signed approximately 160 years after the conclusion of the Earth-Romulan War in 2415, and after the end of the Fourth Romulan Civil War.

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. The treaty also expressly prohibited the development or use of cloaking device technology by the Federation. Then-Captain 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.

While not mentioned on screen, it's 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.

It is possible that the treaty ban only related to the use of cloaking devices on space-faring vessels. On at least one occasion Starfleet created self-replicating mines with cloaking ability, with no mention of Romulan complaints.
Alternatively, the Romulans may have been unconcerned by the violation due to the seriousness of the situation with the Dominion.

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 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. However, Captain Benjamin Sisko and his crew violated this amendment on several occasions by using the cloaking device in the Alpha Quadrant.)

According to Sisko, a Romulan officer was supposed to join the Defiant crew to operate the cloaking device and ensure its safekeeping.

In Q's "anti-time future," the Federation possessed and openly used cloaking technology. The Romulan Empire had been conquered by the Klingon Empire, which presumably nullified the Treaty.

Despite the ban on cloaking technology, various Federation vessels experimented with the technology in the early 2400s. In 2410, the USS Columbia launched with a crippled cloaking system, officially designated as a "stealth system," that allowed it to circumvent the ban.
In 2411, however, Star Fleet allowed a full stealth system, based on Romulan technology, to be installed and used on the vessel, but only for defensive and reconnaissance measures, making the ship Star Fleet's first fully cloaked vessel of the line.

In 2413, the Tal'Shiar seized the Neutral Zone and militarized it, unilaterally ending the Treaty of Algeron and closing the border in the name of security. Forced to act, the government attacked, and the ensuing Fourth Romulan Civil War turned the region into a battlefield as the Tal'Shiar and Uihvueri allied with each other to overthrow the ruling coalition on Eilhuann. After the Tal'Shiar was defeated and disbanded, the Imperial government kept the zone militarized, ending its neutral status and officially pulling out of the Treaty of Algeron, a move tacitly agreed by the Federation in light of the potential for future trouble to erupt in the region again.

During the war, the Imperial Fleet informed the Federation that they knew of the Columbia's cloaking system and ask for the Columbia's help on several sensitive missions inside the Zone.