Pope had withdrawn to the line of…
August 1862 CE
He was aware of Lee's plan because a Union cavalry raid had captured a copy of the written order.
Stuart had nearly been captured during this raid; his famous cloak and plumed hat had not escaped, however, and Stuart retaliates on August 22 with a raid on Pope's headquarters at Catlett's Station, capturing the Union commander's dress coat.
Stuart's raid demonstrates that the Union right flank is vulnerable to a turning movement, although river flooding brought on by heavy rains will make this difficult.
It also reveals the plans for reinforcing Pope's army, which will eventually bring it to the strength of one hundred and thirty thousand men, more than twice the size of the Army of Northern Virginia.