Shiloh... Oh, Shiloh. Peach blossoms, blood-filled ponds, deep woods and open fields... and one small church.
OK. Gear up and strap down - here we go.
US Grant had exploited the Tennessee and Cumberland rivers, knocked out Forts Henry and Donaldson on those rivers, broke a hole in the Confederate line running from the Mississippi River to Bowling Green, KY and - not least - broke the only direct railroad between those points.
In order to counterattack, Albert Sidney Johnston had to pull back to a point on an east-west railroad line - Corinth, in northern Mississippi. He also pulled in the garrisons on the Mississippi River at Columbus, of Mobile, AL and New Orleans, LA, which took time. In the interim, Union forces under Buell took Nashville - a critical industrial, transportation and supply point (IE locomotives, iron-works and horses) and - as a result of stripping the garrison - New Orleans and Memphis would be lost.
Grant was ordered to find a point on the Tennessee River to wait for additional troops, and for Buell's Army to come up and join him. He chose Pittsburgh Landing, and decided to train his troops rather than dig fortifications.
The Confederate troops had assembled at Corinth, a tiny town quite overwhelmed by their numbers, and proceeded to march north. The units had not marched long distances, were not used to working together, and were slowed by rain and mud, so it took three days to move only about 20 to 30 miles. March discipline was non-existent and Confederate troops made a lot of noise, including firing off their muskets, but they were in position on April 5th. Johnston believed their surprise was lost, but resolved to attack anyway on the morning of April 6th.
Johnston's battle plan - to crush the Union left and drive them away from the river and into the wilderness - was sound. The operational plan, drawn up by Beauregard, was a disaster, spreading each of the three corps out over the entire length of the battlefield, one behind the other instead of alongside each other. Union troops had been patrolling, but not far enough out - some surprise was achieved but in general Union troops were able to get into line as the attack came in. Sherman (headquartered near the little Shiloh Church on the Union right) and other officers got their men into line and began conducting a fighting withdrawal toward the river. Grant had been some 20 miles away at Savannah, TN, and arrived mid-morning. Union plans went awry when Wallace's division got lost in the woods and was out of action for most of the day - had they come out as expected they would have hit the Confederates in the left flank and rear, which could have been a game-changer. Grant never forgave Wallace (author of Ben Hur), though it seems to not have been his fault.
One key moment was Grant ordering Prentiss to hold at all costs - his division was finally overwhelmed but it bought the Union precious time. At dusk, Grant lined up the Army artillery on a ridge just short of the river. By that point AS Johnston was dead, the Confederate troops tired and disorganized, and Beauregard ordered a halt for the night rather than make a last desperate assault. I have seen Grant's final position - it is very strong, and with artillery lined up hub-to-hub the casualties in an assault would have been horrific.
Some Union officers, like Sherman, thought the Union army beaten; Grant did not ('Lick 'em tomorrow', he said). He did receive some reinforcements from Buell - about one small division overnight and a bit more during the next day - and counter-attacked on the following morning (April 7th). Confederate units resisted but Union pressure drove them back until it seemed the Confederate army would break, at which point Beauregard pulled his men out and marched back to Corinth.
Part of the shock of Shiloh was that this one battle had more casualties than all of the American Revolution, War of 1812 and Mexican War
combined. Union pre-battle strength was around 45,000 men and Confederate strength about 55,000 men. From the Wiki:
Union casualties were 13,047 (1,754 killed, 8,408 wounded, and 2,885 missing); Grant's army bore the brunt of the fighting over the two days, with casualties of 1,513 killed, 6,601 wounded, and 2,830 missing or captured. Confederate casualties were 10,699 (1,728 killed, 8,012 wounded, and 959 missing or captured). https://en.wikipedia.org/wiki/Battle_of_Shiloh
The armies - and the casualties - were much larger than Manassas, or any other action to date. The green troops either fled or fought until they died - they did not have the veteran knowledge of knowing how to take cover and how to avoid charging a prepared defender. Anti-Grant forces (like Halleck) spread false rumors that he was drunk; just about everyone criticized him for not making a pursuit, which Halleck had forbidden... but, given the situation, it is hard to criticize his handling of the battle. One unintended effect was that reports of him smoking a cigar during the battle led to people sending Grant thousands of cigars; he died of throat cancer.
Shiloh absolutely knocked down the idea that 'one hard battle' would see the North give in or the South fold up; Grant, among others, came away convinced it would be a long and hard war.
The consequences of Shiloh were numerous, and awful for the South. The army had failed to turn back the invaders despite terrible casualties; Nashville would remain in Union hands, Memphis and New Orleans would fall. The Confederate army at Corinth would waste away by disease; the evacuation of Corinth meant the main east-west Confederate railroad line was cut forever. Beauregard's career prospects were almost destroyed; Jefferson Davis never trusted him with anything but garrison duty again.
I strongly - strongly - recommend "Personal Memoirs of US Grant". It contains some of the best, clearest writing on the Civil War - factual, concise, without a lot of self-promotion or justification, and the battles he describes will be absolutely clear to you. McPherson's "Battle Cry of Freedom" is an excellent one-volume history.