The Battle of Shiloh (also known as the Battle of Pittsburg Landing) was an early battle in the Western Theater of the American Civil War, fought April 67, 1862, in southwestern Tennessee. The battle is named after a small church in the vicinity named Shiloh which ironically translates to "place of peace" or "heavenly peace". The Union Army of the Tennessee (Major General Ulysses S. Grant) had moved via the Tennessee River deep into Tennessee and was encamped principally at Pittsburg Landing on the west bank of the Tennessee River, where the Confederate Army of Mississippi (General Albert Sidney Johnston, P. G. T. Beauregard second-in-command) launched a surprise attack on Grant's army from its base in Corinth, Mississippi. Johnston was mortally wounded during the fighting; Beauregard took command of the army and decided against pressing the attack late in the evening. Overnight, Grant was reinforced by one of his divisions stationed farther north and was joined by three divisions from the Army of the Ohio (Maj. Gen. Don Carlos Buell). The Union forces began an unexpected counterattack the next morning which reversed the Confederate gains of the previous day.

On April 6, the first day of the battle, the Confederates struck with the intention of driving the Union defenders away from the river and into the swamps of Owl Creek to the west. Johnston hoped to defeat Grant's army before the anticipated arrival of Buell and the Army of the Ohio. The Confederate battle lines became confused during the fighting, and Grant's men instead fell back to the northeast, in the direction of Pittsburg Landing. A Union position on a slightly sunken road, nicknamed the "Hornet's Nest" and defended by the divisions of Brig. Gens. Benjamin Prentiss and William H. L. Wallace, provided time for the remainder of the Union line to stabilize under the protection of numerous artillery batteries. Wallace was mortally wounded when the position collapsed, while several regiments from the two divisions were eventually surrounded and surrendered. Johnston was shot in the leg and bled to death while leading an attack. Beauregard acknowledged how tired the army was from the day's exertions, and decided against assaulting the final Union position that night.

Tired but unfought and well-organized men from Buell's army and a division of Grant's army arrived in the evening of April 6 and helped turn the tide the next morning, when the Union commanders launched a counterattack along the entire line. Confederate forces were forced to retreat, ending their hopes of blocking the Union advance into northern Mississippi. Though victorious, the Union army had suffered heavier casualties than the Confederates, and Grant was heavily criticized in the media for being taken by surprise.

The Battle of Shiloh was the bloodiest engagement of the Civil War up to that point, with nearly twice as many casualties as the previous major battles of the war combined.

The American Civil War (April 12, 1861 – May 9, 1865; also known by other names) was a civil war in the United States between the Union (states that remained loyal to the federal union, or "the North") and the Confederacy (states that voted to secede, or "the South"). The central cause of the war was the status of slavery, especially the expansion of slavery into territories acquired as a result of the Louisiana Purchase and the Mexican–American War. On the eve of the Civil War in 1860, four million of the 32 million Americans (~13%) were enslaved black people, almost all in the South.The practice of slavery in the United States was one of the key political issues of the 19th century. Decades of political unrest over slavery led up to the Civil War. Disunion came after Abraham Lincoln won the 1860 United States presidential election on an anti-slavery expansion platform. An initial seven southern slave states declared their secession from the country to form the Confederacy. Confederate forces seized federal forts within territory they claimed. The last-minute Crittenden Compromise tried to avert conflict but failed; both sides prepared for war. Fighting broke out in April 1861 when the Confederate army began the Battle of Fort Sumter in South Carolina, just over a month after the first inauguration of Abraham Lincoln. The Confederacy grew to control at least a majority of territory in eleven states (out of the 34 U.S. states in February 1861), and asserted claims to two more. Both sides raised large volunteer and conscription armies. Four years of intense combat, mostly in the South, ensued.

During 1861–1862 in the war's Western Theater, the Union made significant permanent gains—though in the war's Eastern Theater the conflict was inconclusive. On January 1, 1863, Lincoln issued the Emancipation Proclamation, which made ending slavery a war goal, declaring all persons held as slaves in states in rebellion "forever free." To the west, the Union destroyed the Confederate river navy by the summer of 1862, then much of its western armies, and seized New Orleans. The successful 1863 Union siege of Vicksburg split the Confederacy in two at the Mississippi River. In 1863, Confederate General Robert E. Lee's incursion north ended at the Battle of Gettysburg. Western successes led to General Ulysses S. Grant's command of all Union armies in 1864. Inflicting an ever-tightening naval blockade of Confederate ports, the Union marshaled resources and manpower to attack the Confederacy from all directions. This led to the fall of Atlanta in 1864 to Union General William Tecumseh Sherman and his march to the sea. The last significant battles raged around the ten-month Siege of Petersburg, gateway to the Confederate capital of Richmond.

The Civil War effectively ended on April 9, 1865, when Confederate General Lee surrendered to Union General Grant at the Battle of Appomattox Court House, after Lee had abandoned Petersburg and Richmond. Confederate generals throughout the Confederate army followed suit. The conclusion of the American Civil War lacks a clean end date: land forces continued surrendering until June 23. By the end of the war, much of the South's infrastructure was destroyed, especially its railroads. The Confederacy collapsed, slavery was abolished, and four million enslaved black people were freed. The war-torn nation then entered the Reconstruction era in a partially successful attempt to rebuild the country and grant civil rights to freed slaves.

The Civil War is one of the most studied and written about episodes in the history of the United States. It remains the subject of cultural and historiographical debate. Of particular interest is the persisting myth of the Lost Cause of the Confederacy. The American Civil War was among the earliest to use industrial warfare. Railroads, the telegraph, steamships, the ironclad warship, and mass-produced weapons saw wide use. In total the war left between 620,000 and 750,000 soldiers dead, along with an undetermined number of civilian casualties. President Lincoln was assassinated just five days after Lee's surrender. The Civil War remains the deadliest military conflict in American history. The technology and brutality of the Civil War foreshadowed the coming World Wars.