President Lincoln replaced Ambrose Burnside with General Joseph Hooker, who had become known as “Fighting Joe” as a result of his aggressive actions in the Peninsular Campaign. By the spring of 1863 he had built the Army of the Potomac to 120,000 men. In late April, Hooker dispatched soldiers to outflank the Confederate forces stationed at Fredericksburg, a move that appeared promising. However, Hooker hesitated and pulled his soldiers into a defensive position near Chancellorsville. Lee commanded a much smaller force of about 60,000 men. He decided to employ the tactic that had been so successful at Second Bull Run—dividing his army in order to strike the opponent from different directions. The risk was that the Union forces would strike with their overwhelming numerical advantage against the originally small and now even smaller forces under Lee. But Lee had correctly discerned the state of General Hooker`s mind, which despite his nickname had become unnerved, leading to an attitude of excess caution and apprehension. On May 2, "Stonewall" Jackson led one portion of Lee’s army against the Union flank and achieved immediate effects. On the following day, Hooker was leaning against a pillar at the Chancellor House when it was hit by a cannonball. The force of the impact left him temporarily unconscious, but he refused to relinquish command. On May 4, Sedgwick`s Union troops, which were already across the river near Fredericksburg, attacked and drove General Early`s Confederate division towards Chancellorsville. Lee responded by ending his attack on Hooker and sending reinforcements to support Early. Being himself without reinforcements, Sedgwick was forced to retreat. Thus freed to resume his attack on Hooker, Lee now found that the Union forces were pulling back to defensive positions across the Rappahannock River. Many observers regard Chancellorsville as Lee’s most outstanding victory, defeating an army twice the size of his own with some of the most daring tactics in military annals. The cost, however, was immense. Confederate casualties numbered more than 13,000, while the North lost 17,000. The much larger population base of the North enabled them to absorb such losses more easily than the South. Further, the South lost one of its most capable military leaders in “Stonewall” Jackson. He was accidentally shot by one of his own men while returning from a reconnaissance mission. His left arm was amputated and he died of pneumonia several days later. From the Northern perspective, it was clear that the drive to Richmond had been halted once again.