The Battle of Stones River, also known as the Second Battle of Murfreesboro, was a battle fought from December 31, 1862, to January 2, 1863, in Middle Tennessee, as the culmination of the Stones River Campaign in the Western Theater of the American Civil War. Of the major battles of the war, Stones River had the highest percentage of casualties on both sides. Although the battle itself was inconclusive, the Union Army's repulse of two Confederate attacks and the subsequent Confederate withdrawal were a much-needed boost to Union morale after the defeat at the Battle of Fredericksburg, and it dashed Confederate aspirations for control of Middle Tennessee.

Union Maj. Gen. William S. Rosecrans's Army of the Cumberland marched from Nashville, Tennessee, on December 26, 1862, to challenge General Braxton Bragg's Army of Tennessee at Murfreesboro. On December 31, each army commander planned to attack his opponent's right flank, but Bragg struck first. A massive assault by the corps of Maj. Gen. William J. Hardee, followed by that of Leonidas Polk, overran the wing commanded by Maj. Gen. Alexander M. McCook. A stout defense by the division of Brig. Gen. Philip Sheridan in the right center of the line prevented a total collapse, and the Union assumed a tight defensive position backing up to the Nashville Turnpike. Repeated Confederate attacks were repulsed from this concentrated line, most notably in the cedar "Round Forest" salient against the brigade of Col. William B. Hazen. Bragg attempted to continue the assault with the division of Maj. Gen. John C. Breckinridge, but the troops were slow in arriving and their multiple piecemeal attacks failed.

Fighting resumed on January 2, 1863, when Bragg ordered Breckinridge to assault the well-fortified Union position on a hill to the east of the Stones River. Faced with overwhelming artillery, the Confederates were repulsed with heavy losses. Falsely believing that Rosecrans was receiving reinforcements, Bragg chose to withdraw his army on January 3 to Tullahoma, Tennessee. This caused Bragg to lose the confidence of the Army of Tennessee.

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.