Complex sentence: A few days prior to the action on Malvern Hill, McClellan believed that the Army of the Potomac was vastly outnumbered by its Confederate foe, and his fear of being cut off from his supply depot left him cautious and wary.
Split into simpler sentences: A few days prior to the action on Malvern Hill, McClellan had a dilemma. He believed that the Army of the Potomac was vastly outnumbered by its Confederate foe, and his fear of being cut off from his supply depot left him cautious and wary.