His regiment was assigned to the northern defenses of Washington, D.C., from September 1862 until May 1864, when Lt. Gen. Ulysses S. Grant called for more troops to support his Overland Campaign. The 11th Vermont joined the Vermont Brigade after the Battle of the Wilderness, and first saw action at the Battle of Spotsylvania Court House. In his first engagement, on May 18, 1864, Warner was severely wounded, but refused to leave the field until the end of the day. He was sent home on furlough to recover.