In political science, triangular diplomacy is a foreign policy of the United States, developed during the Vietnam War (1955–1975) by Henry Kissinger, as a means to manage relations between the contesting communist powers, the Soviet Union and China. Connecting heavily with the correlating policy of linkage, the policy was intended to exploit the ongoing rivalry between the two Communist powers (following the Sino-Soviet split [1956–1966]), as a means to strengthen American hegemony and diplomatic interest.

Interrelating primarily with the subsequent development of the détente era (1969–1979) during the Cold War, triangular diplomacy was instituted in order to prevent the decline of American authority during the Vietnam War following the perceived inefficiencies of George Kennan's defensive policy of containment and Dwight Eisenhower's offensive policy of rollback. Hence, triangular diplomacy was an instrumental facet in the shifting of Cold War policy toward talks of co-operation and diplomacy, and thus set a precedent for the eventual relaxation of tensions between the two superpowers through a focus on mutual benefit (as evidenced in the Strategic Arms Limitation Talks (SALT) and the Strategic Arms Reduction (START) treaties).

In principle, the policy of triangular diplomacy seeks to link the interests of three powerful states in order to retain a balance of power within the international system. Most commonly, this involves an insecure state (or states) pursuing strategic alliances or economic deals in an attempt to both weaken the hegemony of a powerful adversary and strengthen their own position. First developed by U.S. advisor Henry Kissinger during the Vietnam War era, the policy was argued to be most effective when reliant upon “the natural incentives and propensities of the players.”

Conceived in a period of American political weakness, Kissinger's doctrine argues that foreign policy needed to rely on a combination of diplomacy and military power in order to bring benefits to all relevant players, and subsequently ensure international stability. This doctrine heavily relates to a neorealist analysis of international politics, professed by scholars such as Kenneth Waltz. Kissinger's desires to seek a “coalition” with China in order to stabilise the power of the Soviet Union reflects the neorealist balance of power theory, as insecure states are seeking to bring an equilibrium to the international order in an attempt to bring peace and benefit the relevant actors.

Entering into the White House during the height of the Vietnam War, one of Kissinger's primary intentions with his policy was to gain Soviet and Chinese assistance in softening North Vietnamese troops, and withdrawing from the conflict with dignity. The Sino-Soviet split offered a ripe opportunity for Kissinger and Nixon to enact such a policy, intended by offering integration into the international trade system, scientific and technological resources and a stabilization of bilateral relations. The first instance of this rapprochement and the beginnings of triangular diplomacy within US policy is reflected in a 14 August 1969 National Security meeting, in which Nixon positioned his intentions to victimise China within the Sino-Soviet split and seek to offer assistance.

The following 1972 Beijing and Moscow summits further exacerbated the existing tensions between China and the Soviet Union, allowing Nixon and Kissinger to gain Soviet co-operation on matters deemed important to U.S. foreign policy. This can be examined in the outcome of the 1972 Spring Offensive, as the United States were able to gain diplomatic co-operation in reaching a peace with North Vietnamese forces. Many scholars contend that Nixon's rapprochement with China, as part of the triangular diplomacy framework, was an instrumental facet in the dissolution of the Vietnam War. Historian Raymond Garthoff argues that triangular diplomacy held a significant bearing on the settlement of the Vietnam War, whilst Jussi Hanhimäki contends that the policy played a critical role in bringing about changes in North Vietnamese negotiation strategy.  

However, triangular diplomacy did not ensure immediate stability and peaceful cooperation. The Vietnam War continued in full force for three years after the emergence of the policy, with continued U.S.-Soviet conflict evolving underneath. For Kissinger and Nixon, the policy was secondary to national interests and security concerns, as the nation needed to appear politically powerful against adversaries.

Following the Vietnam War, Kissinger sought to reshape the U.S. approach to international relations, seeking a balance of power which could produce stability and thereby reduce military and political tensions between the three main players in the international order; the Soviet Union, the United States, and China. Triangular diplomacy consequently included the aim of achieving this balance of power and pursuing the policy of détente with the Soviet Union.