I just finished reading several books on Grant, including Chernow's biography, which was a delight because it delves into so much interesting detail about him after the Civil War. As I was reading, I kept coming back to Shiloh.
Shiloh has troubled me for some time. The general consensus about the battle is that the first day was a near-averted disaster, and that Grant's grit on the second day turned the fight into a heavy casualty draw/win.
I believe Shiloh was more than that, and may be the most decisive battle of the war. It is certainly true that Grant was caught on his heels on the first day. It's also correct that the casualties were massive, particularly at this point of the war. Finally, the results of Shiloh were not decisive at the moment, as the Confederate forces were able to withdraw and ultimately regroup.
However, there is a significant question about what would have happened if Grant was not in charge at Shiloh. What if for example, General Buell was leading the forces, or General Halleck. While we will never know, there is every possibility that the Army of Tennessee would have surrendered on the first day, or suffered mass surrender while a rump portion of the army escaped across the river.
At best, Buell or Halleck, if they had survived the first day, would almost certainly have withdrawn their forces, giving the south a massive victory, rather than a costly check. If a massive southern victory had been followed by the Seven Days and Second Bull Run, might the Union have sued for peace? What about 1863? Without Grant's Vicksburg campaign, the inevitable result of Shiloh, would the south have kept the Union on the defense? Could Lee have drawn more forces from a stagnant west, sharpening the victories at Fredricksburg and Chancellorsville, or changing the result at Gettysburg? Would a series of victories in 1862 have led to European recognition, mediation, or even involvement in 1862 or 1863.
While we will never know the answer to these questions, clearly a significant victory at Shiloh would have had the potential to change the course of history. Thus I advocate that Grant's victory at Shiloh, the right person in the right place at the right moment, may have saved the Union cause, and was his most important victory of the war.