Henry Wager Halleck was a 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 General-in-Chief of all Union armies from 1862 to 1864 during the American Civil War.
Early in the Civil War, Halleck was a senior Union Army commander in the Western Theater. He commanded operations in the Western Theater from 1861 until 1862, during which time, while the Union armies in the east were defeated and held back, the troops under Halleck's command won many important victories. However, Halleck was not present at the battles, and his subordinates earned most of the recognition. The only operation in which Halleck exercised field command was the siege of Corinth in the spring of 1862, a Union victory which he conducted with extreme caution. Halleck also developed rivalries with many of his subordinate generals, such as Ulysses S. Grant and Don Carlos Buell. In July 1862, following Major General George B.