The trustees of Columbia University refused to accept Eisenhower's resignation in December 1950, when he took an extended leave from the university to become the Supreme Commander of the North Atlantic Treaty Organization (NATO), and he was given operational command of NATO forces in Europe. Eisenhower retired from active service as an Army general on May 31, 1952, and he resumed his presidency of Columbia. He held this position until January 20, 1953, when he became the President of the United States.