The United States played a leading role in the Bretton Woods and Yalta conferences with the United Kingdom , the Soviet Union , and other Allies , which signed agreements on new international financial institutions and Europe 's postwar reorganization . As an Allied victory was won in Europe , a 1945 international conference held in San Francisco produced the United Nations Charter , which became active after the war . The United States developed the first nuclear weapons and used them on Japan in the cities of Hiroshima and Nagasaki ; causing the Japanese to surrender on September 2 , ending World War II . Parades and celebrations followed in what is known as Victory Day , or V-J Day
international conference
United Nations Conference on International Organization
United States