Here's the amazing story of the Battle of Shiloh
The Union Army under the command of general Ulysses S. Grant, which already had control of most of Tennessee, had been slogging up the Tennessee and Cumberland rivers for months in early 1862. The ultimate objective was the Mississippi and the prospect of seizing control of the river and splitting the Confederacy in two.
Grant had already scored a pair of major victories by taking Forts Henry and Donelson, and the reeling Confederate forces under general Albert Sidney Johnston were forced to gather in the city of Corinth in northern Mississippi, a vital rail center. Grant planned to rendezvous his army of 49,000 with the 20,000 under general Don Carlos Buell and seize Corinth. Johnston meanwhile had assembled an army of 45,000 men in the vicinity of Corinth and was waiting for reinforcements of his own.
Word reached Corinth that Grant was unloading his army at Pittsburg Landing on the Tennessee river 20 miles away, near a small church called Shiloh, Hebrew for "Place of Peace." Johnston's extravagantly named second in command general Pierre Gustave Toutant Beauregard urged an immediate surprise attack. If Grant and Buell's armies combined before battle was joined, there would be little that could stop them in the theatre. Beauregard had a reputation as a tactical expert due to his victory in the war's first major battle at the First Bull Run, and Johnston agreed to the proposal.
The Confederate army advanced on April 3, but was immediately slowed by pouring rain and poor coordination of units along the washed out roads. These same had the same effect on Buell's movement to join Grant, and it became a race between the Confederates and the Union reinforcements floundering through the mud. So terrible were the rains and confusion on the march that the Confederates were forced to delay their attack until April 6.
Albert Sidney Johnson
Grant and his subordinate and best friend general William Tecumseh Sherman did not expect an attack so soon. When Sherman received reports of enemy troops approaching on the morning of April 6th, he first dismissed them as jumpy troops reporting nothing. When he incredulously rode out to see for himself, the Confederates main battle line boiled out of the trees, and the first thing Sherman witnessed was his aide getting shot in the head in front of him. The Union troops were not dug in, and many of them were raw recruits who had only just received their rifles. They were taken completely by surprise. What was worse, Grant was 9 miles downriver staying at a mansion, at least two hours away by boat while his army fought for it's life.
The initial Confederate attack slowly drove the Union army north towards the river, and so intense was the fighting that as many as 10,000 Union troops fled and hid. Grant arrived at around 9 a.m. by steamboat and began to take charge of the defense, but the Union lines gradually collapsed. The fighting began to concentrate on the Union center in a small forest, later called "The Hornet's Nest" for the sheer intensity of the fire directed at the position. An old wagon trail called the Sunken Road that bisected the forest gained it's own infamy as it become completely choked with the dead and wounded of both sides.
When Johnston saw Confederate troops hesitating to join the assault in the face of such slaughter, he personally led a charge that broke a Union strongpoint at a spot later known as the Peach Orchard, with terrific slaughter. While riding back from the successful attack, Johnston was shot in the leg and had his femoral artery severed, leaving him dead in minutes. The resulting lull as Beauregard took command gave the Union army a breather, but soon another Confederate assault resulted in the surrender of an entire Union division and the defense at the Hornet's Nest collapsed.
The surviving Union forces were arranging for a last ditch defense at Snake creek when deliverance arrived in the form of Buell, whose army started crossing the river at sundown. Even so, a final Confederate assault was in the offing when Beauregard, who was unaware of the pending enemy reinforcements, called off the attack until morning. He believed that the Union army was in shambles and only needed to be mopped up. Without the arrival of Buell's army, he may have been right.
The next morning, the bolstered Union army launched a massive counterattack that eventually drove the exhausted and hopelessly outnumbered Confederates from the field, forcing Beauregard to order a retreat back to Corinth with what was left of his army. It was one of the great reversals of the Civil War.
Despite his "victory," Grant faced severe criticism for the laxness of his position and for being away from the army, but despite calls for his resignation President Abraham Lincoln famously said "I can't spare that man. He fights." The South was stunned at the death of Johnston, who was considered the finest soldier in the South, and Confederate president Jefferson Davis was heartbroken at the death of his old West Point classmate.
The scale of the bloodshed at Shiloh left both sides horrified, with nearly 24,000 casualties from both sides. The previous major engagements had been bloody enough, but the United States had never seen a battle with that level of slaughter before. The battle of Shiloh had resulted in more battle casualties than all of America's previous wars combined. As all the terrible battles to follow would attest, Shiloh showed that there was not going to be any cheap, easy victory for either side.