The Photographic History of The Civil War: in ten volumes, Thousands of Scenes Photographed 1861-65, with Text by many Special Authorities, Volume 2: Two Years of Grim War.
Francis Trevelyan Miller, Ed.

Manassas Junction, while his vast store of army supplies was at the latter place.

Pope's great source of uncertainty lay in the fact that he did not know whether Lee would move against him or would follow McClellan in the latter's retreat from the Peninsula; nor did he know when the reenforcements promised from McClellan's army would reach him. Meanwhile Lee had decided to let McClellan depart in peace and to advance against Pope, with the whole Confederate army. To this end Longstreet was ordered to the scene and with his corps he reached Gordonsville on August 13th.

A few days later the two Confederate generals, Lee and Longstreet, ascended to the top of Clark's Mountain, from which, through powerful field-glasses, they obtained a good view of Culpeper, about twelve miles away. They saw that Pope's position was weak and determined to attack him without delay. Lee ordered his army to cross the Rapidan. He also sent a courier to gallop across the country with an important dispatch to General Stuart, disclosing his plans. It was now that General Pope met fortune; he captured the courier and learned of Lee's plans. Pope knew that he was not in position to meet Lee's army at Culpeper, and he withdrew from that place and took up a strong position behind the Rappahannock. Lee had strained every nerve to get at his antagonist before the latter left Culpeper and before he could be reenforced by McClellan's army. But sudden rains changed the Rappahannock from a placid stream into a rushing torrent. The Confederates were delayed and meantime the reenforcements from the Peninsula began to reach Pope's army. General Reno with a part of Burnside's corps was on the ground by August 14th. One week later came Generals Kearny and Reynolds--both splendid leaders, both destined to give their lives for their country within a year — to join the Army of Virginia with some thousands of additional fighters from the Army of the Potomac.