On the morning of April 6, 1862, 40,000 Confederate soldiers under Gen. Albert Sidney Johnston struck the encamped divisions of Union soldiers near Pittsburg Landing on the Tennessee River. The overpowering Confederate attack drove the unprepared Federal soldiers back and threatened to overwhelm Maj. Gen. Ulysses S. Grant’s entire Army of the Tennessee. Some Federal units made determined stands, and by afternoon, had established a battle line at the “Hornet's Nest.” Repeated Rebel attacks supported by massed artillery killed or wounded many of the defending Yankees and pushed their lines back further. Johnston was mortally wounded and was replaced by Gen. Pierre G.T. Beauregard. Fighting continued until after dark, but the Union army held. By the next morning, Grant had been reinforced by the Army of the Ohio under Maj. Gen. Don Carlos Buell and heavily outnumbered Beauregard. Grant launched a counteroffensive along the entire line, overpowering the weakened Confederate forces and driving Beauregard’s army from the field. The Confederate defeat ended any hopes of blocking the Union advance into northern Mississippi. The two-day battle at Shiloh produced more than 23,000 casualties and was the bloodiest battle in American history up to that time.