The Battle of the Crater took place during the American Civil War, part of the Siege of Petersburg. It occurred on Saturday, July 30, 1864, between the Confederate Army of Northern Virginia, commanded by General Robert E. Lee, and the Union Army of the Potomac, commanded by Major General George G. Meade (under the direct supervision of the general-in-chief, Lieutenant General Ulysses S. Grant).

After weeks of preparation, on July 30 Union forces exploded a mine across from Union Major General Ambrose E. Burnside's IX Corps sector, blowing a gap in the Confederate defenses of Petersburg, Virginia. Instead of being a decisive advantage to the Union, this precipitated a rapid deterioration in the Union position. Unit after unit charged into and around the crater, where most of the soldiers milled in confusion in the bottom of the crater. Grant considered this failed assault as "the saddest affair I have witnessed in this war."

The Confederates quickly recovered, and launched several counterattacks led by Brigadier General William Mahone. The breach was sealed off, and the Union forces were repulsed with severe casualties, while Brigadier General Edward Ferrero's division of black soldiers was badly mauled. It may have been Grant's best chance to end the siege of Petersburg; instead, the soldiers settled in for another eight months of trench warfare.

Burnside was relieved of command for his role in the fiasco, and he was never returned to command. Ferrero and General James H. Ledlie were observed behind the lines in a bunker, drinking liquor throughout the battle. Ledlie was criticized by a court of inquiry into his conduct that September, and in December he was effectively dismissed from the Army by Meade on orders from Grant, formally resigning his commission on January 23, 1865.

During the Civil War, Petersburg, Virginia, was an important railhead, where four railroad lines from the south met before they continued to Richmond, Virginia, the capital of the Confederacy. Most supplies to General Lee's army and Richmond funneled through that location. Consequently, the Union regarded it as the "back door" to Richmond and as necessary for its defense. The result was the siege of Petersburg. It was actually trench warfare, rather than a true siege, as the armies were aligned along a series of fortified positions and trenches more than 20 miles (32 km) long, extending from the old Cold Harbor battlefield near Richmond to areas south of Petersburg.

After Lee stopped Grant's attempt to seize Petersburg on June 15, the battle settled into a stalemate. Grant had learned a hard lesson at Cold Harbor about attacking Lee in a fortified position and was chafing at the inactivity to which Lee's trenches and forts had confined him. Finally, Lt. Col. Henry Pleasants, commanding the 48th Pennsylvania Infantry of Major General Ambrose E. Burnside's IX Corps, offered a novel proposal to break the impasse.

Pleasants, a mining engineer from Pennsylvania in civilian life, proposed digging a long mine shaft under the Confederate Army lines and planting explosive charges directly underneath a fort (Elliott's Salient) in the middle of the Confederate First Corps line. If successful, not only would all the defenders in the area be killed, but also a hole in the Confederate defenses would be opened. If enough Union troops filled the breach quickly enough and drove into the Confederate rear area, the Confederates would not be able to muster enough force to drive them out, and Petersburg might fall. Burnside, whose reputation had suffered from his 1862 defeat at the Battle of Fredericksburg and his poor performance earlier that year at the Battle of Spotsylvania Court House, agreed to Pleasants's plan.

Digging began in late June, but even Grant and Meade saw the operation as a "mere way to keep the men occupied" and doubted it of any actual tactical value. They quickly lost interest, and Pleasants soon found himself with few materials for his project, and his men even had to forage for wood to support the structure.