Why was the Secretary of War given a new title?

The Secretary of War was given a new title to fully define his roles and responsibilities. Before the change in entitlement, he was known to be the head of all military affairs and this included the navy. He was also expected to be a successor of the president next to the Secretary of State. Because the National Security Act of 1947 was passed, there was now a defined division of roles namely: the Secretaries of the Army, the Navy, and the Air Force.