23,746 men were killed
16,510 men were wounded
3,844 men were missing in action**
(**scale: Fenway Park, in Boston holds approximately 40, 000 people -- imagine all of the baseball fans at a single game suddenly being killed or wounded and you can grasp just how horrible this was)
Almost everyone began the day believing that the battle would end the Civil War, at least in what was then thought of as the "west."
By the end of the battle, almost everyone knew that the war would be far longer than expected, and far bloodier.
From the National Park Service --
"The Battle of Shiloh began about 4:55 A.M., Sunday, April 6, 1862, when a reconnoitering party of General Benjamin Prentiss’ Union Division was sent out, without authorization, by Col. Everett Peabody. This detachment encountered General William J. Hardee’s skirmish line, under Major Aaron B. Hardcastle more than a mile in front of General William T. Sherman’s camps. The reconnoitering party, three companies of the 25th Missouri and two from the 12th Michigan under Major James E. Powell, fought at Fraley Field and retreated slowly toward their camps under attack from an increasing Confederate force. They were reinforced by five companies of the 21st Missouri and one of the 16th Wisconsin and formed a line at Seay Field, where Col. David Moore took command. Under increasing pressure, and with Moore wounded, they gradually fell back forming with Peabody’s entire brigade at 7:30 A.M. on a ridge about a quarter mile in front of their camps. After making a stand they were forced back to their camp about 8:30A.M. Col. Peabody briefly rallied them until he was killed and they fell back in disorder with the rest of Prentiss’ Division. Prentiss would succeed in rallying about 1,200 of his men along a farm road, later called the “Sunken Road” about a mile to the rear."
(read more from the National Park Service: NPS SHILOH)