Today is the most important day of the war. Today the forces of Grant and Buell will end the Confederate attempt to turn the tide of the war. There will be times that the Confederates make gains again, there will be campaigns into Pennsylvania, Maryland and Kentucky. After the Kentucky campaign the forces will settle into an area farther north of Shiloh (Murfreesboro) but never again will the Confederates be doing this well in the Mississippi River Valley. Grant will make some stumbles along the way to capturing Vicksburg (the overland campaign that ends after the raid on Holly Springs being the largest stumble) but he will not have to fight for this ground again.
I am among those who think the war was decided in the western theater. I think that Virginia was a waste of manpower. That the only way the war could have been decided in Virginia was if Lee had captured Washington, and held it. That ultimately the Confederates should have left as few men in Virginia as needed to prevent the fall of Richmond and sent the rest west to try to defeat Grant along the Mississippi River or Buell/Rosecrans in Tennessee.
As a modern war this war was to be decided by who controlled the resources. Long gone is the time of two armies meeting in a field, slugging it out and the winner of the battle wins the war. Its simplistic to say that the west had more of the resources simply because it was bigger than Virginia but this is true. Virginia was more industrialized so it was a better location to turn the raw materials into usable war goods but it did not have the raw materials in the quantities the South needed.
One big resource was transportation, both by rail and river. Rivers could be a hindrance or a help. In a simplistic sense whoever controlled the rivers found them to be a great help. This is pretty obvious. And the greatest of all America rivers was the Mississippi. It the Confederates controlled it they would restrict Union trade to the rail network of the north, plus whatever portions of the Ohio River they controlled. One big benefit for both sides is the ability to transfer troops long distances, if they controlled the length of the river from Cairo to New Orleans. A force in Vicksburg could be easily sent to meet a problem along the river while the rail network from there would take a much more circuitous route. Of course not all flareups would be along the river but when they were this was a great way to quickly deal with the threat. If the Union controls the river they effectively cut Arkansas, Texas and most of Louisiana off from the rest of the Confederacy. This helps strengthen the blockade as well as deprive the Confederates of home grown supplies, such as cattle, from those states. It also allows the Union several entry points to launch more invasions of the South.
To win a long war the Confederacy needed to control their resources as long as possible. The rivers of the West make this very hard to accomplish. The rivers act as routes of invasion while the rivers in Virginia are much more impediments to an invading army. Grant is the first Union general to seize on this and exploit it with his victories at Forts Henry and Donelson. This opens up much of Kentucky and Tennessee. Johnston is forced to fall back from Bowling Green to Nashville and he quickly abandons that too, eventually retreating to Corinth to prepare for a counterattack with as many men as the Confederates can gather in the west. They strip coastal defenses knowing that they must risk big to stem the Union tide.
And on April 6th it nearly succeeds. Johnston's men surprise Grant's army at Pittsburg Landing and come close to pulling off a huge victory. They drive the Union from every position (some more stubbornly than others), they capture about 2000 men near the end of the day and seem on the verge of ultimate victory when Beauregard orders the final attack stopped. Beauregard has taken over command of the army following the death of Johnston, who was mortally wounded while cheering his men as they made a charge on the Peach Orchard.
We can argue whether or not the final attack had a real chance to succeed or not. I believe it did not but I've heard well reasoned arguments that it wasn't a foregone conclusion that it would fail. But in any respect the attack is not made, Grant's men finally get to rest after a hellish day of combat and Buell's army begins to arrive from a long march from Nashville. On April 7th the Union forces will have plenty of fresh men while the Confederates will have none. The fighting on April 7th won't be without drama but in hindsight the Confederates could only fight for a stalemate. They do pretty well but at the end of the day they know they must retreat to Corinth, that their attempt has failed.
Grant and Buell will not press their advantage on the 8th. And Halleck will take about 4 weeks to cover the 20 miles to Corinth, assuring the Confederates of a chance to escape that monstrous army group. The war would continue for three more years but on April 7th the Confederates lost their last opportunity to turn the tide.