On May 4, 1864, Grant began a series of battles with Robert E. Lee and the Army of Northern Virginia known as the Overland Campaign. The first battle between Lee and Grant took place after the Army of the Potomac crossed Rapidan River into an area of secondary growth trees and shrubs known as the Wilderness. Lee was able to use this protective undergrowth to counter Grant's superior troop strength. Union Maj. Gen. Winfield S. Hancock's II corps was able to inflict heavy casualties and drive back the Confederate General A.P. Hill's corps two miles; however, Lee was able to drive back the Union advance with Confederate General James Longstreet's reserves. The difficult, bloody, and costly battles lasted two days, May 5 and 6, resulting in an advantage to neither side. Unlike Union generals who retreated after similar battles with Lee, Grant ignored any setbacks and continued to flank Lee's right moving southward. The tremendous casualties for the Battle of the Wilderness were 17,666 for the Union and 11,125 for the Confederate armies, respectively.