Halleck, Henry Wager (1815–1872), a United States army officer. During 1862–64 Halleck was general in chief of the Union armies in the Civil War. Well versed in the theory of warfare, he was called “Old Brains.” He failed to develop a winning strategy, however, and became mainly a channel of communications between President Lincoln and the field commanders. In March, 1864, General U. S. Grant was given command of the Union armies and Halleck became his chief of staff, retaining that post until the war's end.

Halleck was born in Westernville, New York. He entered the army after graduation from West Point in 1839. During the Mexican War he was breveted a captain. In 1854 Halleck resigned from the army to become a lawyer. He was made a major general at the start of the Civil War and was in charge of Union troops in the west until July, 1862, when he became general in chief although his actual rank remained major general. After the war Halleck was commander of the Division of the Pacific, 1865–69, and the Division of the South, 1869–72.