5th New York Infantry on May 14, 1861. Warren and his regiment saw their first combat at the Battle of Big Bethel in Virginia on June 10, arguably the first major land engagement of the war. He was promoted to colonel and regimental commander on September 10. In the 1862 Peninsula Campaign, Warren commanded his regiment at the Siege of Yorktown and also assisted the chief topographical engineer of the Army of the Potomac, Brig. Gen. Andrew A. Humphreys, by leading reconnaissance missions and drawing detailed maps of appropriate routes for the army in its advance up the Virginia Peninsula. He commanded a small brigade (3rd Brigade, 2nd Division, V Corps) during the Seven Days Battles consisting of his own 5th New York along with the 10th New York. At Gaines Mill, he was bruised in the knee by a shell fragment, but remained on the field. He continued to lead the brigade at the Second Battle of Bull Run, suffering heavy casualties in a heroic stand against an overwhelming enemy assault, and at Antietam, where the V Corps was in reserve and saw no combat. Warren was promoted to brigadier general on September 26, 1862, and he and his brigade went to the Battle of Fredericksburg in December, but again were held in reserve and saw no action. When Maj. Gen. Joseph Hooker reorganized the Army of the Potomac in February 1863, he named Warren his chief topographical engineer and then chief engineer. As chief engineer, Warren was commended for his service in the Battle of Chancellorsville. At the start of the Gettysburg Campaign, as Confederate General Robert E. Lee began his invasion of Pennsylvania, Warren advised Hooker on the routes the Army should take in pursuit. On the second day of the Battle of Gettysburg, July 2, 1863, Warren initiated the defense of Little Round Top, recognizing the importance of the undefended position on the left flank of the Union Army, and directing, on his own initiative, the brigade of Col. Strong Vincent to occupy it just minutes before it was attacked. Warren suffered a minor neck wound during the Confederate assault. Promoted to major general after Gettysburg (August 8, 1863), Warren commanded the II Corps from August 1863 until March 1864, replacing the wounded Maj. Gen. Winfield S. Hancock, and distinguishing himself at the Battle of Bristoe Station. On March 13, 1865, he was brevetted to major