In early May, the Army of the Potomac crossed the Rapidan River and entered the area known as the Wilderness of Spotsylvania. There, in dense woods that nullified the Union army's advantages in artillery, Robert E. Lee surprised Grant and Meade with aggressive assaults. The two-day Battle of the Wilderness was tactically inconclusive, although very damaging to both sides. However, unlike his predecessors, Grant did not retreat after the battle; he sent his army to the southeast and began a campaign of maneuver that kept Lee on the defensive through a series of bloody battles and moved closer to Richmond. Grant knew that his larger army and base of manpower in the North could sustain a war of attrition better than Lee and the Confederacy could. And although Grant suffered high losses—approximately 55,000 casualties—during the campaign, Lee lost even higher percentages of his men, losses that could not be replaced.