Shiloh was the first battle that showed the country how bloody the Civil War actually would be. It's a fascinating encounter between
Joseph E. Albert Sydney Johnston an U.S. Grant in which Johnston was killed. You can read up on the battle here. Anyway, here's Grant discussing the notion that the Confederate army would have won Shiloh "if" Johnston wasn't killed:
I do not question the personal courage of General Johnston, or his ability. But he did not win the distinction predicted for him by many of his friends. He did prove that as a general he was over-estimated. General Beauregard was next in rank to Johnston and succeeded to the command, which he retained to the close of the battle and during the subsequent retreat on Corinth, as well as in the siege of that place. His tactics have been severely criticised by Confederate writers, but I do not believe his fallen chief could have done any better under the circumstances. Some of these critics claim that Shiloh was won when Johnston fell, and that if he had not fallen the army under me would have been annihilated or captured.Ifs defeated the Confederates at Shiloh. There is little doubt that we would have been disgracefully beaten IF all the shells and bullets fired by us had passed harmlessly over the enemy and IF all of theirs had taken effect. Commanding generals are liable to be killed during engagements; and the fact that when he was shot Johnston was leading a brigade to induce it to make a charge which had been repeatedly ordered, is evidence that there was neither the universal demoralization on our side nor the unbounded confidence on theirs which has been claimed. There was, in fact, no hour during the day when I doubted the eventual defeat of the enemy, although I was disappointed that reinforcements so near at hand did not arrive at an earlier hour.