In March 1941, Roosevelt sent Harriman to London as "defense expediter" to coordinate and facilitate the anticipated flood of American wartime supplies to Britain under the newly established Lend-Lease program. Harriman quickly established a warm and confidential relationship with Britain's wartime prime minister, Winston L. S. Churchill. He often bypassed the U.S. ambassador, John G. Winant, to report directly to White House aide Harry Hopkins and serve as an unofficial liaison between Churchill and Roosevelt. In London, Harriman, although married, began a clandestine love affair with Churchill's daughter-in-law, Pamela Digby Churchill, who eventually (in 1971) became his third wife.
When Germany invaded Russia in June 1941, the United States extended Lend-Lease aid to the Soviet Union, and dealings with Moscow likewise fell within Harriman's remit. In 1943, Harriman replaced Admiral William H. Standley as U.S. ambassador to the Soviet Union, where he remained until 1946, attending the major wartime Allied conferences at Tehran, Yalta, and Potsdam. In late 1944, when Soviet troops allowed occupying German forces to suppress Polish rebels in Warsaw before themselves mopping up the remaining Germans (a policy deliberately designed to eliminate future opponents of a Soviet-backed Polish regime), Harriman sent diplomatic dispatches to Washington sounding one of the earliest official warnings against future Soviet designs for the nations of eastern Europe.
Harriman subsequently held numerous other government positions under Democratic presidents, gaining the reputation of advocating a firm but flexible and nonalarmist stance toward the Soviets. Harriman died in Yorktown Heights, New York, on 26 July 1986.
Abramson, Rudy. Spanning the Century: The Life of W. Averell Harriman, 1891–1986. New York: Morrow, 1991.; Bland, Larry. "W. Averell Harriman: Businessman and Diplomat, 1891–1945." Ph.D. dissertation, University of Wisconsin, 1972.; Harriman, W. Averell, and Elie Abel. Special Envoy to Churchill and Stalin, 1941–1946. New York: Random House, 1975.; Isaacson, Walter, and Evan Thomas. The Wise Men: Six Friends and the World They Made. New York: Simon and Schuster, 1986.; Mayers, David. The Ambassadors and America's Soviet Policy. New York: Oxford University Press, 1995.