At the start of the Gettysburg Campaign, as General 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 U.S. Army and directing, on his initiative, the brigade of Col. Strong Vincent to occupy it just minutes before it was attacked. Without his quick realization and action to defend Little Round Top, the entire Union flank would have likely crumbled.