Truman appointed Clayton as the first Under Secretary of State for Economic Affairs, a post Clayton held for 1946-47. In this role, Clayton witnessed the weakness of U.S. allies and their susceptibility to falling under the domination of the Soviet Union. He urged greater U.S. engagement with the world to halt the spread of communism. In a March 5 memo, Clayton wrote a fifteen-point manifesto for U.S. global leadership, in which he argued:"The reins of world leadership are fast slipping from Britain's competent, but now very weak hands. These reins will be picked up either by the United States or by Russia. If by Russia, there will almost certainly be a war in the next decade or so, with the odds against us. If by the United States, war can almost certainly be prevented." Clayton strongly supported American economic aid to rebuild Europe after World War II and had a major role in shaping the Marshall Plan in 1947. After returning from a meeting at the United Nations Economic Commission for Europe ECE in Geneva in May, Clayton wrote a memo to George Marshall, entitled "The European Crisis," in which he argued that U.S. economic aid was urgently needed to prevent the collapse of Europe. In this memo, he warned that "without further prompt and substantial aid from the United States, economic, social and political disintegration will overwhelm Europe Charles Bohlen, when drafting the announcement of the Marshall Plan, drew heavily from Clayton's memo."