The history of the defense of the United States started with the Continental Congress in 1775. The creation of the United States Army was enacted on 14 June 1775. This coincides with the American holiday Flag Day. The Second Continental Congress would charter the United States Navy, on 13 October 1775, and create the United States Marine Corps on 10 November 1775.
The Preamble of the United States Constitution gave the authority to the federal government to defend its citizens:
We the People of the United States, in Order to form a more perfect Union, establish Justice, insure domestic Tranquility, provide for the common defence, promote the general Welfare, and secure the Blessings of Liberty to ourselves and our Posterity, do ordain and establish this Constitution for the United States of America.
— Constitution of the United States
Upon the seating of the first Congress on 4 March 1789, legislation to create a military defense force stagnated as they focused on other concerns relevant to setting up the new government. President George Washington went to Congress to remind them of their duty to establish a military twice during this time. Finally, on the last day of the session, 29 September 1789, Congress created the War Department, historic forerunner of the Department of Defense. The War Department handled naval affairs until Congress created the Navy Department in 1798. The secretaries of each of these departments reported directly to the president as cabinet-level advisors until 1949, when all military departments became subordinate to the Secretary of Defense.
National Military Establishment
President Harry Truman
signs the National Security Act Amendment of 1949
After the end of World War II, President Harry Truman proposed creation of a unified department of national defense. In a special message to Congress on 19 December 1945, the President cited both wasteful military spending and inter-departmental conflicts. Deliberations in Congress went on for months focusing heavily on the role of the military in society and the threat of granting too much military power to the executive.
On 26 July 1947, Truman signed the National Security Act of 1947, which set up a unified military command known as the "National Military Establishment", as well as creating the Central Intelligence Agency, the National Security Council, National Security Resources Board, United States Air Force (formerly the Army Air Forces) and the Joint Chiefs of Staff. The act placed the National Military Establishment under the control of a single Secretary of Defense. The National Military Establishment formally began operations on 18 September, the day after the Senate confirmed James V. Forrestal as the first Secretary of Defense. The National Military Establishment was renamed the "Department of Defense" on 10 August 1949 and absorbed the three cabinet-level military departments, in an amendment to the original 1947 law.
Under the Pub.L. 85–599), channels of authority within the department were streamlined, while still maintaining the ordinary authority of the Military Departments to organize, train and equip their associated forces. The Act clarified the overall decision-making authority of the Secretary of Defense with respect to these subordinate Military Departments and more clearly defined the operational chain of command over U.S. military forces (created by the military departments) as running from the president to the Secretary of Defense and then to the unified combatant commanders. Also provided in this legislation was a centralized research authority, the Advanced Research Projects Agency, eventually known as DARPA. The act was written and promoted by the Eisenhower administration, and was signed into law 6 August 1958.