Why was the Battle of Shiloh eye opening

It was a very bloody battle and they took them by surprise.

The Union had 13,000 casualties

The Confederate had 10,700 casualties

On the morning of April 6, 1862, the Confederate Army of the Mississippi under Johnston launched an attack on Maj. Gen. Grant's Army of the Tennessee near Pittsburg Landing. One author has even gone so far as to call it the Pearl Harbor of the Civil War. In actuality, Shiloh was not all that much of a surprise.