The Seven Days' Battles were important because, despite heavy losses, the Confederate army under Robert E. Lee not only prevented George B. McClellan's Union army from capturing Richmond, but also drove them into retreat down the Virginia Peninsula. Instead of McClellan's Peninsula Campaign offensive bringing the Civil War to a swift conclusion, the war went on for three more years.
The Seven Days' Battles occurred from June 25 to July 1, 1862. The Battle of Oak Grove on June 25 cost both sides over 1,000 casualties and enabled the Union army to advance only 600 yards. The following day, Lee attacked at Beaver Creek Dam, but due to poor coordination, the Confederate army did not achieve its objectives. Inexplicably, however, McClellan began to retreat. After the Battle of Gaines' Mill, a major Confederate attack, McClellan abandoned his plan to take Richmond and ordered his army to withdraw, despite having substantial remaining forces and a strong position.
The Battle of Garnett's and Golding's Farm further convinced McClellan that he was being attacked on all sides and had to move his army out of danger. During the Battles of Savage's Station, Glendale, White Oak Swamp and Malvern Hill, the Confederates continued to harass but not destroy the Union army. Before the last battle, McClellan went north and left the fighting to his subordinates.
In the course of the Seven Days' Battles, Lee's army suffered 20,000 casualties compared to 16,000 for the Union army. Despite the heavy losses, Confederate confidence was bolstered, while the defeat and retreat had a devastating effect on Union morale.