The Treaty of Alliance was the official agreement that established the alliance between the United Federation of Planets and the Klingon Empire. According to the treaty's terms, each side could request the other's assistance in fighting against its enemies. (TNG: "Redemption, Part I") Vulcan ambassador Sarek was one of the key figures behind the negotiations that created the alliance. (TNG: "Sarek")
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. The Romulan Star Empire was driven 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, Part II")
By contrast, 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")
- The Treaty of Alliance may have been signed around the same time as the Second Khitomer Accords which were mentioned in Star Trek: Insurrection. Various DS9 episodes seemed to equate the Khitomer Accords with the Treaty of Alliance, implying that they were connected in some way.