George Brinton McClellan (December 3, 1826 October 29, 1885) was an American soldier, Civil War Union general, civil engineer, railroad executive, and politician who served as the 24th governor of New Jersey. A graduate of West Point, McClellan served with distinction during the MexicanAmerican War (18461848), and later left the Army to serve as an executive and engineer on railroads until the outbreak of the American Civil War (18611865). Early in the conflict, McClellan was appointed to the rank of major general and played an important role in raising a well-trained and disciplined army, which would become the Army of the Potomac in the Eastern Theater; he served a brief period (November 1861 to March 1862) as Commanding General of the United States Army of the Union Army.

McClellan organized and led the Union army in the Peninsula Campaign in southeastern Virginia from March through July 1862. It was the first large-scale offensive in the Eastern Theater. Making an amphibious clockwise turning movement around the Confederate Army in northern Virginia, McClellan's forces turned west to move up the Virginia Peninsula, between the James River and York River, landing from Chesapeake Bay, with the Confederate capital, Richmond, as their objective. Initially, McClellan was somewhat successful against General Joseph E. Johnston, but the emergence of General Robert E. Lee to command the Army of Northern Virginia turned the subsequent Seven Days Battles into a Union defeat. However, historians note that Lee's victory was in many ways pyrrhic as he failed to destroy the Army of the Potomac and suffered a bloody repulse at Malvern Hill.

General McClellan and President Abraham Lincoln developed a mutual distrust, and McClellan was privately derisive of Lincoln. McClellan was removed from command in November in the aftermath of the 1862 midterm elections. A major contributing factor in this decision was McClellan's failure to pursue Lee's army following the tactically inconclusive but strategic Union victory at the Battle of Antietam outside Sharpsburg, Maryland.

McClellan never received another field command and went on to become the unsuccessful Democratic Party nominee in the 1864 presidential election against the Republican Lincoln. The effectiveness of his campaign was damaged when he repudiated his party's platform, which promised an end to the war and negotiations with the Confederacy. He served as the 24th Governor of New Jersey from 1878 to 1881; he eventually became a writer, and vigorously defended his Civil War conduct.

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.