The Defense Department was established after World War II by an act of Congress, meaning that Trump likely lacks the authority to unilaterally change its official name.

To avoid that issue, the White House said the president is authorizing the use of the new label as a "secondary title" by his administration.

Here is a look at key facts about the history of the departments overseeing the United States military.

- More than 200 years old -

The War Department was established in August 1789 to oversee the US Army, Navy and Marine Corps, according to an official Pentagon history web page.

A little less than a decade later, responsibility for naval forces was transferred to the new Department of the Navy, which also gained responsibility for the US Marine Corps in 1834.

That left the War Department responsible for the Army, and later for the Army Air Corps -- the precursor of the Air Force.

- Reorganized after WWII -

The War Department underwent a major reorganization -- and name change -- following World War II.

The changes began with the signing of the National Security Act by then president Harry Truman in July 1947.

The legislation merged the War and Navy Departments as well as the Air Force into the "National Military Establishment" led by a defense secretary.

The National Security Act was amended in August 1949 to change the name of the National Military Establishment to the Department of Defense.

It also removed the secretaries of the Army, Navy and Air Force from cabinet positions and made them subordinate to the defense secretary.

- Headquartered at the Pentagon -

The US Defense Department is headquartered at the Pentagon, a massive five-sided building located in Virginia on the Potomac River.

Construction of the building was authorized due to space constraints in the run-up to World War II, with personnel of what was then known as the War Department split between more than a dozen buildings at the time and the number of staff expected to grow.

Ground was broken on the Pentagon in September 1941 and it officially opened 16 months later in January 1943. The Defense Department is now the largest employer in the United States, with more than three million military and civilian personnel.