In June 1864, Confederate Gen. Robert E. Lee detached Gen. Jubal A. Early's corps from the Richmond battlefields and sent it to the Shenandoah Valley to oppose Union Gen. David Hunter's army. After driving Hunter into West Virginia, Early invaded through Maryland toward Washington, D.C., to draw Union troops from Richmond and free Confederate prisoners at Point Lookout. On July 9, he sent Gen. Bradley T. Johnson's cavalry brigade, including Maj. Harry Gilmor, east toward Baltimore, while Union Gen. Lew Wallace delayed Early at the Battle of Monocacy as Federal reinforcements strengthened Washington's defenses. Early briefly probed those defenses on July 11-12, then withdrew to the Shenandoah Valley and halted the Federal pursuit at Cool Spring on July 17-18, and although he failed to take Washington, the invasion diverted Federal resources. At Island (Parker's) Ford about 3:30 P.M. on July 18, 1864, Union Col. George D. Wells's brigade, leading Col. Joseph Thoburn's division, crossed the Shenandoah River to the western side in pursuit of Early's corps, which had recrossed the river the day before. Part of Confederate Maj. Jesse Richardson's 42nd Virginia Infantry tried to resist the crossing, but Wells's brigade overpowered them and captured twelve Virginians, one captain and eleven enlisted men, while securing the ford. After Union Gen. George Crook learned from the prisoners that Early's full force was nearby, he urged the overall Union commander, Gen. Horatio G. Wright, to withdraw Thoburn's regiments at once. Wright refused and instead ordered Gen. James B. Ricketts's division to support Thoburn, but although Ricketts arrived around 6:00 P.M., he did not cross the river because he and Wright believed the battle was lost. Ricketts's failure to support Thoburn remained a lasting grievance among many of Thoburn's veterans even decades after the war ended.