The United States Department of War, also called the War Department (and occasionally War Office in the early years), was the United States Cabinet department responsible for the operation and maintenance of the United States Army. It also had responsibility for naval affairs from 1794 until the establishment of the Department of the Navy in 1798, and for most non-naval air forces until the creation of the Department of the Air Force on September 18, 1947.

The secretary of war, a civilian with such responsibilities as finance and purchases and a minor role in directing military affairs, headed the War Department throughout its existence.

The War Department existed for 158 years, from August 7, 1789, to September 18, 1947, when, under the National Security Act of 1947, it split into the Department of the Army and the Department of the Air Force (which, together with the Department of the Navy, formed the National Military Establishment). In 1949, the National Military Establishment was renamed the Department of Defense.

The Department of War traces its origins to the committees created by the Second Continental Congress in 1775 to oversee the Revolutionary War. Individual committees were formed for each issue, including committees to secure ammunition, to raise funds for gunpowder, and to organize a national militia. These committees were consolidated into the Board of War and Ordnance in 1776, operated by members of Congress. A second board was created in 1777, the Board of War, to operate separately from Congress. The Congress of the Confederation eventually replaced the system of boards with the Department of War. Only five positions were created within the department upon its creation: the secretary at war, an assistant, a secretary, and two clerks.

Shortly after the establishment of a government under President George Washington in 1789, Congress reestablished the War Department as a civilian agency to administer the field army under the president (as commander-in-chief) and the secretary of war. Retired senior General Henry Knox, then in civilian life, served as the first United States secretary of war. When the department was created, the president was authorized to appoint two inspectors to oversee the troops. Congress created several additional offices over the course of the 1790s, including the major general, brigadier general, quartermaster general, chaplain, surgeon general, adjutant general, superintendent of military stores, paymaster general, judge advocate, inspector general, physician general, apothecary general, purveyor, and accountant.

Forming and organizing the department and the army fell to Secretary Knox, while direct field command of the small Regular Army fell to President Washington.^{[citation needed]} In 1798, Congress authorized President John Adams to create a second provisional army under the command of former president Washington in anticipation of the Quasi-War, but this army was never utilized. The Department of War was also responsible for overseeing interactions with Native Americans in its early years.

On November 8, 1800, the War Department building with its records and files was consumed by fire.

The United States Military Academy at West Point and the Army Corps of Engineers were established in 1802. The Department of War was reduced in size following the end of the Quasi-War in 1802, but it was subsequently expanded in the years leading up to the War of 1812. To accommodate this expansion, sub-departments were created within the department, with each one led by a general staff officer. These sub-departments were reformed into a modern system of bureaus by Secretary of War John C. Calhoun in 1818. Secretary Calhoun created the Bureau of Indian Affairs in 1824, which served as the main agency within the War Department for addressing the issues regarding Native Americans until 1849, when Congress transferred it to the newly founded Department of the Interior. The U.S. Soldiers' Home was created in 1851.