Answer
US President Lincoln concluded that General McClellan did not leave enough troops to defend Washington DC during the Peninsula warfare. Lincoln, therefore, managed troops from McClellan's army, the Army of the Potomac to guarantee the Federal capital was protected well. McClellan had attained nearly all of the Army of the Potomac into Virginia, and as explained later in the war, a defined Southern attack could force a large contingent to rapidly return to Washington to preserve the capital. The capital cities of Richmond and Washington are only about 100 miles apart.
