Monnet developed his initial contacts with William Clayton during the time of the Lendlease programmes when Clayton was assistant administrator of lend-lease as part of his portfolio as Assistant Secretary of Commerce. Clayton had also worked with Dean Acheson in the United Nations Relief and Rehabilitation Agency (UNNRA) on supply and financial problems of the allies and so Monnet would have had contact with him at this time and so Clayton remained part of Monnet's Washington network which he relied on to advance his plans for Europe. In 1945 Clayton was appointed Assistant Secretary of State for Economic Affairs, where he was concerned with European economic recovery, and it was with Clayton that Monnet negotiated loans for the French recovery programme. In 1946, Acheson got authority from Congress to create a temporary post for Clayton, that of Under-Secretary of State for Economic Affairs. Within this position, Clayton assumed responsibility for the Foreign Economic Administration (FEA), which meant he was dealing with the difficult task of the reconstruction of Europe. It was Clayton that raised the alarm bells in the US, about the need for aid under new terms and conditions, so as to prevent war in coming decades. He saw first hand the terrible conditions in post-war Europe and the need for economic recovery before political integration could be tackled. So Clayton had a big part in the thinking behind the Marshall Plan, for in two memoranda, he outlined the underestimation of the destruction of the European economy by the war. Clayton also highlighted the importance of including Germany in Marshall aid as a necessary step in the recovery of Europe. In the summer of 1947, Monnet held a series of meetings with Clayton, who at that time was the senior American official in Europe. Their intensive rethinking of policies to be adopted with Germany were behind Monnet's ideas in the Schuman Plan. They saw the linkage between the recovery of France and Western Europe and the future of Europe in their ideas for an international authority for the Ruhr in which Germany would play a part. Clayton was also one of the chief contributors to the International Trade Organisation, established to boost the European economy.

The originals are held in various Archives and Presidential Libraries in the USA