Final answer:
The reorganization of the war department in 1943 eventually led to the Department of Defense's establishment, incorporating the Army, Navy, and Air Force under one umbrella following the National Security Act of 1947.
Step-by-step explanation:
The office established in 1943 as a result of reorganizing the war department was the precursor to what would become part of the Department of Defense after a series of reorganizations and legislative actions. Significant changes to U.S. military structure would emerge following the end of World War II, culminating in the passage of the National Security Act of 1947. This act was responsible for streamlining government security measures, creating entities like the Central Intelligence Agency (CIA) and the National Security Council, and leading to the formation of the Department of the Air Force. In 1949, the Departments of the Army, Navy, and the newly created Air Force would be unified under the Department of Defense, transforming the U.S. military's organizational and command structure.