On the morning of April 6, 1862, 40,000 Confederate soldiers under the command of Gen. Albert Sidney Johnston poured out of the woods and struck the encamped divisions of Union soldiers occupying ground near Pittsburg Landing on the Tennessee River. The overpowering Confederate attack drove the unprepared Federal soldiers from their camps 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 a sunken road, known as the “Hornet's Nest.” Repeated Rebel attacks supported by massed artillery failed to carry the Hornet's Nest but killed, or wounded many of defending Yankees. During the first day’s fighting, Johnston was mortally wounded and was replaced by Gen. Pierre G.T. Beauregard. Fighting continued until after dark, but the Union line held. By the next morning, the Federals had been reinforced by the Army of the Ohio under Maj. Gen. Don Carlos Buell and numbered about 54,000 men, outnumbering Beauregard’s army of around 30,000. 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.