Treaty of Alliance
The Treaty of Alliance (also known as the Klingon Alliance) was the official agreement that established the alliance between the United Federation of Planets and the Klingon Empire in 2352. Vulcan Ambassador Sarek was one of the key figures behind the negotiations that created the alliance, and his son, Ambassador Spock, was the one who proposed the alliance. (VOY: "Alliances") The Treaty was regarded with controversy when first suggested, but later proved to be a cornerstone for peace in the Beta Quadrant.
The Federation-Klingon alliance represented a fundamental shift in the balance of power in the Alpha and Beta Quadrants, as the two former enemies united. According to the treaty's terms, each side could request the other's assistance in fighting against its enemies. This clause, in part, forced the Romulan Star Empire to the sidelines, unable to compete against the combined forces of the Federation and the Klingons. As a result, the Romulans devoted considerable resources towards trying to break up the alliance, albeit without success. (TNG: "Reunion", "The Mind's Eye", "Redemption", "Redemption II")
Some of the stipulations included in the treaty were unrestricted travel within each others borders and the ability to conduct safety and health inspections aboard each others ships that passed through.
The treaty was effectively broken in 2372 upon the Klingon invasion of Cardassian space, which the Federation denounced. The resulting Second Federation-Klingon War lasted just over a year, until it was discovered that the Dominion had manipulated circumstances surrounding the invasion. The treaty was re-imposed shortly afterward.