|Abstract||This study was conducted to discover the ideas and importance of William C. Bullitt during his tenure as the United States Ambassador to France, 1936-1940. Specifically it endeavored to learn what policies he suggested, and why he thought as he did. It also sought to evaluate his influence on the development of American foreign policy in the years concerned. The sources used were limited, but rich. The correspondence between Ambassador Bullitt and President Franklin D. Roosevelt, contained in the Franklin D. Roosevelt Library at Hyde Park, provided the greatest information. Also very helpful were the Louis B. Wehle Papers and the R. Walton Moore Papers also located at Hyde Park. The official diplomatic correspondence published in the Foreign Relations series proved most valuable during crisis periods when events moved too rapidly for communication by courier. The Edward M. House Collection at Yale University was essential for understanding Bullitt's early thinking. several diaries and memoirs, published by Bullitt's contemporaries , helped fill the remaining gaps. These various sources revealed that Bullitt underwent a vast transformation in outlook during his four years in Paris. Beginning with an isolationist-revisionist bias, he supported appeasement in the expectation that it might encourage Franco-German reconciliation, which he considered vital to prolonged peace in Europe. During the German-Czechoslovak Crisis, from May through August, 1938, he continued to favor appeasement, but more as a means for securing the time needed to rearm the Democracies, although he still sought reconciliation. A few days before the Munich conference Bullitt began shifting his position dramatically, and by its termination he strongly opposed further appeasement of Germany. He maintained this policy henceforth. Bullitt's altered perspective caused him to become increasingly concerned with America's role in Europe. Following the Munich conference he strongly urged Roosevelt to mobilize the industrial capacity of the United States for the purpose of providing the British and French with whatever war materiel they needed, especially airplanes. He successfully won Roosevelt's support for this program, although it never proved as successful as had been hoped. Bullitt remained deeply involved with the formulation of American foreign policy until the fall of France in June, 1940. Following his return to the United States in July he and the President drew apart, and his ability to shape official policy decreased. Although Bullitt struggled valiantly to reestablish his influence within the administration, his efforts proved futile, and he was forced to abandon public life.