Final answer:
The Department of Defense was established by the National Security Act of 1947 and reorganized in 1949 to incorporate the Army, Navy, Air Force, and Marine Corps under a single executive component. This was part of a strategic reorganization to ensure efficient military command and alignment with civilian leadership as outlined in the U.S. Constitution.
Step-by-step explanation:
The authorization for the creation of the Department of Defense (DoD) came with the National Security Act of 1947. This act was pivotal in reorganizing U.S. military and security agencies post-World War II, establishing the National Security Council and the Central Intelligence Agency (CIA), and creating the Department of the Air Force. The DoD as we know it today, with its executive component that includes the Army, Navy, Air Force, and Marine Corps, was formed in 1949 as part of a reorganization that merged the Department of War (established in 1789), the Department of the Navy (established in 1798), and the newly created Department of the Air Force into a single department to provide an integrated command structure for the Armed Services.
While the Constitution, specifically Article II, establishes the President as Commander in Chief of the military forces, the actual structure and organization of these forces have evolved through legislation over time. The passage of the National Security Act and the formal creation of the DoD underscored America's commitment to containing Soviet expansion during the early years of the Cold War. It is clear that the Founders intended for military forces to be under civilian control, with the President overseeing them and Congress holding the 'power of the purse' to fund military actions.