In 1950 Thompson went to Rome and was then appointed high commissioner and ambassador to Austria (1952–1956), his most productive years as a Cold War diplomat. His tenacious skills as a negotiator contributed both to a settlement of the Trieste issue in 1954 and the signing of the Austrian State Treaty in May 1955, both milestones in East-West diplomacy. He went on to serve during 1957–1961 as ambassador to Moscow, where he secured Soviet leader Nikita Khrushchev's personal trust. This served Thompson well as an advisor to Presidents Dwight D. Eisenhower and John F. Kennedy during Cold War crises over Berlin and Cuba between 1958 and 1962.
Thompson facilitated Khrushchev's visit to America in 1959 as well as U.S.-Soviet summits in Paris (1960) and Vienna (1961). Thompson returned to Washington in 1962 and was appointed ambassador-at-large. During the 1962 Cuban Missile Crisis, he exerted a moderating influence in White House Executive Committee meetings. Involved in many crucial nuclear arms talks, President Lyndon B. Johnson appointed Thompson ambassador to Moscow for a second time in 1966, a post he held until 1969, making him the longest-serving ambassador to Moscow in American history. Thompson died in Bethesda, Maryland, on 6 February 1972.
Bischof, Günter. Austria in the First Cold War, 1945–1955: The Leverage of the Weak. London: Macmillan, 1999.; Garthoff, Raymond L. A Journey through the Cold War: A Memoir of Containment and Coexistence. Washington, DC: Brookings Institution Press, 2001.; Mayers David. The Ambassadors and America's Soviet Policy. New York: Oxford University Press, 1995.