The Battle of Shiloh began on April 6, 1862 as an effort by Confederate forces to stop Union troop movements along the Tennessee River. 40,000 troops under the command of two of the most aggressive Confederate Generals, P.T.Beauregard and A.S.Johnston attacked the Union army under the command of Ulysses S. Grant while they were camped in an area near Shiloh Meeting House, near Pittsburg Landing (which is another name the battle has been known as).
Grant, who was waiting on reinforcements from Nashville under the command of Don Carlos Buell, was caught by surprise and suffered heavy casualties. The advance of the Confederate forces on April 6th was slowed before they could reach the river where Grant's troops were camped with heavy cannon fire and desperate efforts by the Union army to avoid annihilation.
The advance of the Confederate troops was slowed enough to give Buell's troops to arrive from Nashville on April 7th. The Union forces now outnumbered the Confederates and Buell's fresh troops overwhelmed the exhausted Confederates, causing them to retreat and allowing Grant and Buell to continue to control their campaign on the Tennessee River.
One of the most curious events of the Battle of Shiloh was the death of Confederate General A.S.Johnston. A very aggressive and fearless commander, Johnston suffered a leg wound at Shiloh. It was not considered to be a very serious injury, but Johnston neglected the wound. It is believed that had he simply agreed to allow a tourniquet to be used, he would have lived. His death disenchanted his troops, speeding their retreat from Shiloh, and allowed Grant's stranglehold on the western theatre of the war to become that much stronger.