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. Vulcan ambassador Sarek (TNG: "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 Alpha 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")
The Treaty allowed the Federation to conduct health and safety inspections of Klingon vessels within Federation space. It also allowed for unrestricted travel by each side in the other's space. (TNG: "Gambit, Part II")
In 2372, the Dominion was able to break the alliance by manipulating the Klingon Empire into an unprovoked invasion of the Cardassian Union, an action condemned by the Federation. The alliance was resumed the following year, after the Dominion joined the Cardassians' war against the Klingons. (DS9: "The Way of the Warrior", "By Inferno's Light")
During the war, the Dominion worked with the Orion Syndicate on another way to break the alliance. By using Klingon weapons to assassinate an anti-Federation ambassador, it was hoped Gowron would be implicated and the Klingon people would call for the alliance to end. (DS9: "Honor Among Thieves")