Wallace's most notable service came on Saturday, July 9, 1864 at the Battle of Monocacy part of the Valley Campaigns of 1864. Although Confederate General Jubal A. Early and an estimated 15,000 troops defeated Wallace's troops at Monocacy Junction, Maryland, forcing them to retreat to Baltimore, the effort cost Early a chance to capture Washington, D.C. Wallace's men were able to delay the Confederate advance toward Washington for an entire day, giving the city time to organize its defenses. Early arrived in Washington at around noon on July 11, two days after defeating Wallace at Monocacy, the northernmost Confederate victory of the war, but Union reinforcements had already arrived at Fort Stevens to repel the Confederates and force their retreat to Virginia.