Despite the confusion that gripped the Army of the Potomac during its August 1862 withdrawal from the outskirts of Richmond, one thing remained clear.

So critical a role did the deep-water port at Yorktown play in Maj. Gen. George B. McClellan's failed Peninsula Campaign that — even after retreating down the James River to Fort Monroe — he issued repeated commands urging that the run-down bastion be rebuilt and defended.

"Push the work laid out by the engineer officers with the utmost rapidity," McClellan ordered on Aug. 22, while waiting to depart from Old Point Comfort.

"It is a matter of vital importance that the (work) details be furnished and the work done in the shortest possible time," he added later that day.

"It is imperatively necessary that the work required should be pushed forward with the utmost vigor."

What the Union got when the soldiers of the 4th Corps finished was a strong forward outpost that repeatedly confirmed McClellan's vision.

Fort Yorktown not only provided a crucial harbor for the Federal blockade of the York River and the Chesapeake Bay but also a springboard for expeditions to the Middle Peninsula and Richmond.

It also became a vital magnet and refuge for thousands of runaway slaves, depriving the Confederacy of badly needed food supplies and labor.

"Yorktown posed a constant threat to Richmond both because of its proximity by land and the ease with which you could send large numbers of troops from its wharves to the Richmond & York River Railroad on the Pamunkey River," historian John V. Quarstein says, describing the historic Civil War stronghold that Colonial National Historical Park will recreate Saturday and Sunday through the living history programs of "Fort Yorktown Sesquicentennial Weekend."

"It would have been even more important had the North had more enterprising commanders and used its resources better."

Fortified by Confederate commander John B. Magruder as part of his 12-mile-long Warwick-Yorktown Defensive Line, the earthen ramparts ringing the old colonial town fell into disrepair soon after besieged Southern forces withdrew in early May 1862.

By late August, their condition was so bad that Union Maj. Gen. John J. Peck described them as "overgrown with noxious weeds, and the earth is saturated with human feces and decaying animal matter.

"The exterior ditch and exterior face of the work is in many places so much washed away and defaced as to form but little impediment to an assaulting column."

All that changed within weeks under Peck's supervision. But it wasn't until 1863 that the Union's investment began to pay off.

That May, Yorktown became the launching pad for more than 30,000 troops as they gathered from across Hampton Roads and shipped out to West Point for Maj. Gen. John A. Dix's Peninsula Campaign, which threatened Richmond in response to Robert E. Lee's march on Gettysburg.

Four months later, Maj Gen. Isaac J. Wistar led a combined force of several thousand infantry, cavalry and artillerymen as well as a dozen Union gunboats in an expedition across the York to Mathews County, where they turned the countryside upside-down in a punishing house-to-house search for Confederate blockade runners and merchant raiders.

Despite severe injuries that limited the use of his arms, Wistar proved so enterprising in his command that he also used Yorktown as the springboard for an ambitious raid on Richmond in February 1864, only to find himself bottled up at Bottoms Bridge because of a Union deserter's warning. He later moved out to help recover the retreating elements of the Kilpatrick-Dahlgren Raid on Richmond in early March, then joined the 33,000-man Army of the James as they passed through Yorktown in May for the Bermuda Hundred Campaign on the James River.

"It was like island-hopping in the Pacific during World War II," says Lee Hall Mansion curator J. Michael Moore.

"Yorktown was the place that you leap-frogged from whenever you wanted to threaten Richmond or the Middle Peninsula — and the importance of that role has been overlooked."

Defended by a garrison of as many as 4,000 infantry, artillery and cavalrymen, the fort sometimes boasted more troops than the Department of Virginia headquarters at Fort Monroe.