As stated at Civilwar.org:
"If I possessed the necessary munitions, I should be unable to supply provisions for the troops. I therefore determined, while threatening the approaches to Washington, to draw the troops into Loudoun, where forage and some provisions can be obtained, menace their possession of the Shenandoah Valley, and, if found practicable, to cross into Maryland. The purpose, if discovered, will have the effect of carrying the enemy north of the Potomac, and, if prevented, will not result in much evil. "
Lee entered Maryland with approximately 65,000 men, and by September 7, he had concentrated most of them in Frederick, a city just forty miles to the northwest of Washington, D.C. On the same day, under orders to protect Washington and Baltimore, and to drive the Confederates back across the Potomac, McClellan led a hastily organized force out of Washington.
The Army of the Potomac comprised elements of three armies, assorted independent commands, and 18,000 new troops. Hampered by poor logistical support and a lack of accurate intelligence reports, McClellan moved cautiously. At Frederick, Lee had positioned himself at a crossroads, with access to both Washington and Baltimore. In order to defend these cities, McClellan was forced to spread his men all the way from the Potomac River to Baltimore.
For further reading visit Encyclopediavirginia.org.
Tip! Check out the biography of General Robert E. Lee - including history articles, links, recommended books and more at Civilwar.org.