Antietam on September 17, but Lee's retreat back to Virginia gave the Union a strategic victory in the campaign. During October and November, Union President Abraham Lincoln pressured McClellan to launch an aggressive campaign against Lee but McClellan refused, instead moving slowly and demanding supplies. Lincoln replaced McClellan on November 7 with Major General Ambrose Burnside, who hoped to cross the Rappahannock River near Fredericksburg, Virginia in order to get between Lee and Richmond. However, delays in obtaining a pontoon bridge prevented Burnside from crossing the river until December 11, by which time Lee was able to concentrate his entire army along a series of ridges near Fredericksburg. On December 13, Burnside attacked the Confederate positions and lost heavily; two days later he retreated back across the river and went into winter quarters. In the Western Theater, the Confederate forces, commanded by General Albert S. Johnston, were forced to abandon Kentucky and much of central and western Tennessee following the loss of Fort Donelson and Fort Henry in February. The capture of the two forts turned the Union commander in the battles, Major General Ulysses S. Grant, into a national hero. Johnston, following a plan proposed by his second-in-command General P. G. T. Beauregard, concentrated as many forces as he could near Corinth, Mississippi and attacked Grant's Army of the Tennessee near Shiloh Church. Although successful in driving the Union army almost into the Tennessee River on April 6, Johnston was mortally wounded during the battle, while Grant was reinforced during the night by the Army of the Ohio, commanded by Major General Don Carlos Buell. Grant then led a counterattack the following morning and drove the Confederates from the field, who then retreated back to their base at Corinth. Major General Henry W. Halleck took command of the Union forces operating in western Tennessee and advanced to Corinth, where both armies settled in for a month-long siege. Fearing that a full scale Union assault on the Confederate defenses was imminent, Beauregard evacuated Corinth during the night of May 29–30 without Halleck's forces finding out until the following morning. Beauregard was relieved of command shortly afterwards, due to his health; Jefferson Davis replaced him with General Braxton Bragg. Following the Union victory at Corinth, Halleck dispersed his army across northern Mississippi, western Tennessee, and northern Alabama to protect the railroads, while sending Buell's Army of the Ohio eastward to capture