The following is an extract of letter from Halleck to McClellan, dated the 6th of August, 1862, explaining the reason for the order for the removal of the troops from Harrison's Landing to Aquia Creek.
Allow me to allude to a few of the facts in the case. You and your officers, at our interview, estimated the enemy's force around Richmond at 200,000 men. Since then you and others report that they have and are receiving large reinforcements from the South. General Pope's army, now covering Washington, is only about 40,000. Your effective force is only about ninety thousand. You are about thirty miles from Richmond, and General Pope eighty or ninety, with the enemy directly between you, ready to fall with his superior numbers upon one or the other, as he may elect.