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 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. (TNG: "Reunion", "The Mind's Eye", "Redemption, Part II")
- 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.