Short answers, pulled from the story.
The Battle of Shiloh was fought on April 6-7, 1862, in southwestern Tennessee, between Shiloh Church and Pittsburg Landing on the Tennessee River. The battlefield is located in what was the Western Theater of the American Civil War.
Major General Ulysses S. Grant commanded the Union forces. The Confederate Army of Mississippi was commanded by General Albert Sidney Johnston, who was killed on the 6th of April 1862, at 2:30 pm, and replaced by General P. G. T. Beauregard. Major General Don Carlos Buell commanded the Army of the Ohio, which reinforced Grant overnight.
Total casualties at Shiloh were approximately 23,746, making it the bloodiest battle in American history up to that point. Union casualties totaled 13,047 and Confederate casualties totaled 10,699, though at least one historian believes both figures are understated.
The Hornet's Nest was a position along an old sunken wagon track between Duncan Field and a peach orchard at the Hamburg-Savannah Road, where Union forces under Prentiss and W.H.L. Wallace held off repeated Confederate attacks for much of the 6th of April 1862. Confederate soldiers named it the Hornet's Nest because of the ferocity of Union fire. Historians estimate between eight and fourteen separate Confederate infantry charges were launched there, involving around 10,000 Confederate soldiers.
Johnston bled to death from a torn popliteal artery in his right leg on the 6th of April 1862, about 100 yards south of the Bell Farm at 2:30 pm. His personal physician had been sent elsewhere to treat the wounded, and a tourniquet might have saved his life. He became the highest-ranking soldier killed in combat in the American Civil War.
Grant was heavily criticized in the press and by politicians after Shiloh, with false rumors claiming he had been drunk during the battle. Major General Henry Halleck arrived on the 11th of April 1862, took personal command, and on April 30 named Grant his second-in-command, a position that served as a de facto suspension. President Lincoln defended Grant, reportedly saying "I can't spare this man; he fights." Grant later led the Siege of Vicksburg, where nearly 30,000 Confederate troops surrendered on the 4th of July 1863.