From the Document: "The Department of Defense (DOD) was established after World War II through the 1947 National Security Act. At the time, some, including President Truman, took the view that the different components of the U.S. military had been insufficiently integrated to wage World War II effectively. The intention of the 1947 Act was therefore to create, for the first time, an integrated institution that combined the Departments of War and Navy, and to establish a policy architecture for overseeing the newly reorganized military apparatus. Over time, DOD has grown into one of the largest bureaucracies in the world, comprising over 3 million employees stationed across the United States and around the globe."
CRS In Focus, IF10543
Congressional Research Service: https://crsreports.congress.gov/