American Civil War: Henry Halleck becomes general-in-chief of the Union Army.

The American Civil War was a civil war in the United States between the Union and the Confederacy, which was formed in 1861 by states that had seceded from the Union. The central conflict leading to war was a dispute over whether slavery should be permitted to expand into the western territories, leading to more slave states, or be prohibited from doing so, which many believed would place slavery on a course of ultimate extinction.

Henry Wager Halleck was a senior United States Army officer, scholar, and lawyer. A noted expert in military studies, he was known by a nickname that became derogatory: "Old Brains". He was an important participant in the admission of California as a state and became a successful lawyer and land developer. Halleck served as the General-in-Chief of the Armies of the United States from 1862 to 1864, and then became Chief of Staff for the remainder of the war when Ulysses S. Grant was appointed to that position.

Commanding General of the United States Army was the title given to the service chief and highest-ranking officer of the United States Army, prior to the establishment of the Chief of Staff of the United States Army in 1903. During the American Revolutionary War (1775–1783), the title was Commander-in-Chief of the Continental Army. Between 1783 and 1821, there was no true overall commander for the army. Historians use the term Senior Officer of the United States Army to refer to the individual that held the highest rank by virtue of his date of commission, though the authority they exerted depended on the will of the Secretary of War. In 1821, Secretary John C. Calhoun appointed Jacob Brown as the Commanding General of the United States Army, thus establishing the office of Commanding General. The office was often referred to by various other titles, such as "Major General Commanding the Army" or "General-in-Chief".