The Battle of Cold Harbor was fought during the American Civil War near Mechanicsville, Virginia, from May 31 to June 12, 1864, with the most significant fighting occurring on June 3. It was one of the final battles of Union Lt. Gen. Ulysses S. Grant's Overland Campaign, and is remembered as one of American history's bloodiest, most lopsided battles. Thousands of Union soldiers were killed or wounded in a hopeless frontal assault against the fortified positions of Confederate Gen. Robert E. Lee's army.

On May 31, as Grant's army once again swung around the right flank of Lee's army, Union cavalry seized the crossroads of Old Cold Harbor, about 10 miles northeast of the Confederate capital of Richmond, Virginia, holding it against Confederate attacks until the Union infantry arrived. Both Grant and Lee, whose armies had suffered enormous casualties in the Overland Campaign, received reinforcements. On the evening of June 1, the Union VI Corps and XVIII Corps arrived and assaulted the Confederate works to the west of the crossroads with some success.

On June 2, the remainder of both armies arrived and the Confederates built an elaborate series of fortifications 7 miles long. At dawn on June 3, three Union corps attacked the Confederate works on the southern end of the line and were easily repulsed with heavy casualties. Attempts to assault the northern end of the line and to resume the assaults on the southern were unsuccessful.

Grant said of the battle in his Personal Memoirs, "I have always regretted that the last assault at Cold Harbor was ever made. ... No advantage whatever was gained to compensate for the heavy loss we sustained." The armies confronted each other on these lines until the night of June 12, when Grant again advanced by his left flank, marching to the James River. In the final stage, Lee entrenched his army within besieged Petersburg before finally retreating westward across Virginia.

The Overland Campaign, also known as Grant's Overland Campaign and the Wilderness Campaign, was a series of battles fought in Virginia during May and June 1864, in the American Civil War. Lt. Gen. Ulysses S. Grant, general-in-chief of all Union armies, directed the actions of the Army of the Potomac, commanded by Maj. Gen. George G. Meade, and other forces against Confederate Gen. Robert E. Lee's Army of Northern Virginia. Although Grant suffered severe losses during the campaign, it was a strategic Union victory. It inflicted proportionately higher losses on Lee's army and maneuvered it into a siege at Richmond and Petersburg, Virginia, in just over eight weeks.

Crossing the Rapidan River on May 4, 1864, Grant sought to defeat Lee's army by quickly placing his forces between Lee and Richmond and inviting an open battle. Lee surprised Grant by attacking the larger Union army aggressively in the Battle of the Wilderness (May 5–7), resulting in heavy casualties on both sides. Unlike his predecessors in the Eastern Theater, however, Grant did not withdraw his army following this setback, but instead maneuvered to the southeast, resuming his attempt to interpose his forces between Lee and Richmond. Lee's army was able to get into position to block this movement. At the Battle of Spotsylvania Court House (May 8–21), Grant repeatedly attacked segments of the Confederate defensive line, hoping for a breakthrough, but the only results were again heavy losses for both sides.

Grant maneuvered again, meeting Lee at the North Anna River (Battle of North Anna, May 23–26). Here, Lee held clever defensive positions that provided an opportunity to defeat portions of Grant's army, but illness prevented Lee from attacking in time to trap Grant. The final major battle of the campaign was waged at Cold Harbor (May 31 – June 12), in which Grant gambled that Lee's army was exhausted and ordered a massive assault against strong defensive positions, resulting in disproportionately heavy Union casualties. Resorting to maneuver a final time, Grant surprised Lee by stealthily crossing the James River, threatening to capture the city of Petersburg, the loss of which would doom the Confederate capital. The resulting siege of Petersburg (June 1864 – March 1865) led to the eventual surrender of Lee's army in April 1865 and the effective end of the Civil War.

The campaign included two long-range raids by Union cavalry under Maj. Gen. Philip Sheridan. In a raid toward Richmond, Confederate cavalry commander Maj. Gen. J.E.B. Stuart was mortally wounded at the Battle of Yellow Tavern (May 11). In a raid attempting to destroy the Virginia Central Railroad to the west, Sheridan was thwarted by Maj. Gen. Wade Hampton at the Battle of Trevilian Station (June 11–12), the largest all-cavalry battle of the war.