One of General Lee's first actions was to order James Ewell Brown Stuart to scout out Union positions. Stuart led his 1,200 cavalry troopers, on a three day ride around Union lines. In his spectacular ride, Stuart completely circled Union forces.
Lee knew that he had to proceed before McClellan could begin a siege of Richmond. On June 26th, Lee attacked at Mechanicsville, the far flank of the Union line. The brunt of the attack was made by the forces of A.P. Hill. It was a lopsided fight. The Confederates sustained 1,500 casualties, while the Union sustained only 400. McClellan, however, felt that with overwhelming Confederate forces, liable to attack, he should withdraw the forces to a more defensible position at Gaines Mill.
The next day Lee attacked again. The battle went on for the whole day. By the end of the day, Confederate forces succeeded in breaking through Union lines at Turkey Hill. When the results of the battle became known, McClellan made the decision to give up his base of supply at the White House, and move it to Harrison's landing. He gave orders for all his forces to retreat to the James River. McClellan, even though his losses had been lower than the Confederate losses to date, was convinced he had lost.