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.

The 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. In 1783, the title was simplified to Senior Officer of the United States Army. In 1821, the title was changed to Commanding General of the United States Army. The office was often referred to by various other titles, such as "Major General Commanding the Army" or "General-in-Chief".