This policy was written as NSC-68, a classified report issued by the United States National Security Council in April 1950 and written by Paul Nitze, Kennan's successor as Director of Policy Planning.[42] Kennan and Charles Bohlen another State Department expert on Russia, fought over the wording of NSC-68, which emerged as the blueprint for waging the Cold War.[43] Kennan rejected the idea that Stalin had a grand design for world conquest implicit in Nitze's report and argued that he actually feared overextending Russian power. Kennan even argued that NSC-68 should not have been drafted at all, as it would make U.S. policies too rigid, simplistic and militaristic. Acheson overruled Kennan and Bohlen, backing up the view of the Soviet menace in NSC-68.[44]
Kennan opposed the building of the hydrogen bomb and the rearmament of Germany, which were policies backed up by the assumptions of NSC-68.[45][46] During the Korean War (which began when North Korea invaded South Korea in June 1950), when rumors started circulating in the State Department that plans were being made to advance beyond the 38th parallel into North Korea, a move that Kennan considered highly dangerous, he engaged in intense arguments with Assistant Secretary of State for the Far East Dean Rusk, who apparently supported Acheson's goal to forcibly unite the Koreas.[47]
Kennan lost influence with Acheson, who in any case relied much less on his staff than Marshall had. Kennan resigned as director of policy planning in December 1949 but stayed in the department as counselor until June 1950.[48] Acheson replaced Kennan with Nitze in January 1950, who was far more comfortable with the calculus of military power. Afterwards, Kennan accepted an appointment as Visitor to the Institute for Advanced Study from fellow moderate Robert Oppenheimer, Director of the Institute.[49]
Despite his influence, Kennan was never really comfortable in government. He always regarded himself as an outsider and had little patience with critics. W. Averell Harriman, the U.S. ambassador in Moscow when Kennan was deputy between 1944 and 1946, remarked that Kennan was "a man who understood Russia but not the United States".[50]