The Army of the Potomac didn't just let Lee retreat. Unfortunately, since Lee got so badly mauled at Gettysburg, historians never truly covered the pell mell retreat of Lee's, known now in historical retrospective. The Union would have considered it a bad wound, but still a dangerous army. Would he retreat and lay a trap? It was Lee in 1863; not Lee in 1865.
Meade would have known Lee was damaged, but how damaged and where was Lee retreating?
Meade kept ready contact using his cavalry. How was Meade to know Lee went on a virtual death march, with his wounded, right up to the Potomac River, only stopping because of high water. A fact, not publicized by historians, because it was Lee. It took three days after Gettysburg, for Meade to find out what Lee did - a rapid retreat.
HAGERSTOWN AND SHARPSBURG PIKE,
Midway between the two places, July 7, [1863]
(Received 11.45 p. in.)
GENERAL: I attempted to take Williamsport yesterday, but found
too large a force of infantry and artillery. After a long fight, I with-
drew to this place. Heavy forces were coming into Williamsport
all night. There are a good many wagons at Williamsport. There
is no bridge there. Troops and wagons are being ferried across in
two flat-boats very slowly. I can do nothing with the enemy save
observe him. There is nothing at Sharpsburg.
I am, very respectfully, your obedient servant,
JNO. BUFORD,
Brigadier-General of Volunteers, Comdg. First Division.
Major-General FRENCH.