At the outbreak of World War II, he was financial advisor of the Embassy of France in Washington, D.C. Opposed to the Vichy regime, he resigned in 1941, and joined Charles de Gaulle in London.
He was then appointed National Commissioner for the Economy, Finance and the Colonies and Director of Economic Affairs of the French Committee of National Liberation (CFLN), first in London and then in Algiers, and became a close advisor to De Gaulle.
He was representative of France to the sixteen nation Conference in Paris, in July 1947, which developed the Marshall Plan.
This included explaining the war in Algeria in the context of decolonization, and with the return of De Gaulle to power in 1958, justifying the French position on NATO, which resulted in the withdrawal of France from the integrated military command of the organization in 1966.
During their stay in Washington, and his wife Nicole Alphand (ex-wife of Stephen Bunau-Varilla), whom he married in 1958, made the Embassy of France renowned for diplomatic receptions during the Kennedy administration.