The United States Department of War, often known as the War Department (and occasionally the War Office in its early years), was the United States Cabinet department in charge of the operation and upkeep of the United States Army. It was also in charge of naval affairs from 1794 to the foundation of the Department of the Navy in 1798, as well as most non-naval air forces until the Department of the Air Force was established on September 18, 1947.

Throughout its existence, the US War Department was led by a civilian, the secretary of war, who was in charge of finance and acquisitions as well as having a small role in military affairs.

The War Department existed for 158 years, from August 7, 1789 to September 18, 1947, when it was divided into the Department of the Army and the Department of the Air Force (which, along with the Department of the Navy, comprised the National Military Establishment). In 1949, the National Military Establishment was renamed the Department of Defence.

When was the U.S. Department of War Created?

On August 7, 1789, during the first session of Congress, President George Washington signed legislation to establish the US Department of War. The Department of War, often known as the War Office (a word taken from the British), was principally responsible for the army. In 1798, the Navy established its own Cabinet-level department, which also handled the Marine Corps.

The first secretary of war was Henry Knox, a close adviser to Washington during the Revolutionary War who had also served as secretary of war under the Articles of Confederation. The new Department of War succeeded the Board of War and Ordnance, which Washington formed in 1776 to oversee the military throughout the Revolution.

What’s the significance of the National Security Act of 1947?

To battle on land, sea, and air in World War II, new levels of cooperation were required across the many branches of the United States military. After the war, when the Soviet Union emerged as a danger, President Harry Truman enacted the National Security Act of 1947, which significantly restructured U.S. military administration.

First, the act abolished the Department of War and established the Department of the Army. Next, it established the United States Air Force as a separate arm from the Army. The four arms of the military were then united into a single organisation known as the National Military Establishment.

According to the 1947 statute, the head of the National Military Establishment (NME) was the secretary of defence, a new Cabinet-level job. The defence secretary was required to be a civilian or resigned from the military for ten years (later reduced to seven). Former Navy Secretary James Forrestal served as the first secretary of defence.

The National Security Act of 1947 established other key components of US national security, such as the Joint Chiefs of Staff, the Central Intelligence Agency, and the National Security Council.

When did it become the Department of Defense?

The National Military Establishment did not last long. Just two years later, Truman approved a series of changes to the National Security Act, establishing the Department of Defense. The problem with the National Military Establishment was that the secretary of defence, who was technically in control, lacked the authority to adjudicate disagreements among the military branches. That’s because the Army, Navy, and Air Force secretaries remained on the president’s Cabinet.

The Department of Defense was established as a new executive agency by revisions to the National Security Act in 1949. Furthermore, the secretaries of the Army, Navy, and Air Force were removed from the Cabinet, while the secretary of defence was granted complete “direction, authority, and control” over the new department, according to Truman.

The National Security Act was amended to incorporate the appointment of a Chairman of the Joint Chiefs of Staff who would provide direct military advice to both the president and the secretary of defence. The first chairman, General Omar Bradley, was appointed in 1949.

In 2025, President Donald Trump signed an executive order reviving the Department of War as a “secondary title” for the Department of Defense. Only Congress can authorise a permanent name change.

The declaration was made Friday during a ceremony in the Oval Office, when Defense Secretary Pete Hegseth was presented as “Secretary of War”, and Deputy Secretary Steve Feinberg was dubbed “Deputy Secretary of War.”