Warren provided important service during the Chancellorsville campaign, but he won his greatest glory at the battle of Gettysburg. On the second day of the battle Warren was sent, at his own request, to examine the left of the Union lines. He proceeded to Little Round Top, a commanding promontory at the base of the Union position, and discovered no troops upon it. He observed that the advancing Confederate forces would outflank the Union line and seize Little Round Top, which would render the Union position at Gettysburg untenable. Warren dispatched several staff officers to secure troops to defend the hill. They were able to obtain a single brigade, which was rushed to the hill, arriving only minutes before the Confederates. A desperate struggle ensued, and the Federal brigade was hard-pressed to hold its position. Warren personally rode to some nearby troops and convinced them to move to the hill's defense. Their arrival turned the tide of the conflict at that critical point.
D. Scott Hartwig, "Warren, Gouverneur Kemble," American National Biography Online, February 2000, http://www.anb.org/articles/05/05-00822.html. .