 At the end of June , Lee began a series of attacks that became known as the Seven Days Battles . The first major battle , at Mechanicsville , was poorly coordinated by Lee and his subordinates and resulted in heavy casualties for little tactical gain . However the battle had a significant impact on McClellan 's nerve . The surprise appearance of Maj. Gen. Stonewall Jackson 's troops in the battle ( when they had last been reported to be many miles away in the Shenandoah Valley ) convinced McClellan that he was even more outnumbered than he had thought . He reported to Washington that he faced 200 @,@ 000 Confederates ( the actual number was 85 @,@ 000 . ) 
