In the Western world, the Sino–Soviet split transformed the bi-polar cold war into a tri-polar cold war, a geopolitical event as important as the erection of the Berlin Wall (1961), the defusing of the Cuban Missile Crisis (1962), and the end of the Vietnam War (1955–1975), because the rivalry facilitated Mao's realisation of Sino–American rapprochement with the 1972 Nixon visit to China. Moreover, the occurrence of the Sino–Soviet split also voided the concept of Monolithic Communism, the Western perception that the communist nations were collectively a unitary actor in post–Second World War geopolitics, especially during the 1947–1950 period in the Vietnam War, when the US intervened to the First Indochina War (1946–1954).[6] Historically, the Sino–Soviet split facilitated the Marxist–Leninist Realpolitik with which Mao established the tri-polar geopolitics (PRC–USA–USSR) of the late-period Cold War (1956–1991) as well as the quad-polar geopolitics (PRC-UK-USA-USSR) until the Suez Crisis of 1956.[7]