As a brigadier general in the Massachusetts militia, Butler entered the war as an officer. He was the first to be appointed a volunteer major general on May 16, 1861. He failed in his defense of Fort Monroe, but he led assaults on the North Carolina coastline. In 1863, he received command of the Army of the James. He served as commander until November 1864 when Grant sent him home to await orders. Butler resigned his commission in November 1865.