Henry Wager Halleck (January 16, 1815 - January 9, 1872) was a major general in the United States Army and Chief of Staff of the Union Army during the Civil War (1862-1864). Later in the military, he was known as the "Old Think Tank".

After graduating from West Point Military Academy in 1839, he later served as an engineer officer. In 1844, he went to Europe to study military science. After the outbreak of the Mexican War, he served in the Pacific Coast Force and served as the Secretary of State of California. He is best known not for his military achievements but for his role in promoting California as a state. In 1854, he resigned from the military and entered the legal profession. After the outbreak of the Civil War, he returned to the army as a major general and served as the Supreme Commander of the Western Battlefield. His subordinate was Ulysses Simpson Grant, who was famous for being a drunkard. He didn't like Grant very much. He asked his adjutants to contact Grant, but Grant led his troops to win battles such as Fort Henry and Island 10. He wanted to pick on Grant's minor faults, but the president liked Grant very much. In 1862, he was appointed Chief of Staff and Army Commander by Abraham Lincoln. In 1664, he served as Chief of General Staff until the end of the war. He was always cautious and carefully prepared for any battle. As an army manager and organizer, he was still very qualified. Author of "Principles of Military Art and Science".