Ilovestars26 ( talk ) 02:12 , 16 May 2011 ( UTC ) After pursuing Confederate General Robert E. Lee into Maryland , Union Army Maj. Gen. George B. McClellan launched attacks against Lee 's army , in defensive positions behind Antietam Creek . After pursuing Confederate General Robert E. Lee into Maryland , Union Army Maj. Gen. George B. McClellan launched attacks against Lee 's army , in defensive positions behind Antietam Creek .