Map by Hampton Newsome, from The Siege of Petersburg: The Battles for the Weldon Railroad, August 1864 (Savas Beatie, 2015)
Grant's launch of his fourth offensive at Petersburg on August 14, 1864, turned the tide in the Siege of Petersburg. It was the most important action between the failure of the initial assaults (June 15-18, 1864) and the final breakthrough of the Federals to the Appomattox River above the city (April 2, 1865).
Why?
Arrival of the Federals at Petersburg's gates pinned Lee. But in early July he was reaping the benefits of dispatching Early's Corps toward Washington, DC. Early's threat to Washington shifted the initiative to Lee. Grant had to dispatch most of his cavalry, VI Corps, and part of XIX Corps to the Shenandoah Valley to safeguard Washington.
Lee reinforced Early's success by sending additional forces to northern Virginia in early August: Kershaw's infantry division and Fitzhugh Lee's cavalry division, as well as some artillery.
As of August 14, Hampton's cavalry division was also on its way to northern Virginia. Field's infantry division had orders to go there. Arrival of those units would put almost as many Confederates in the neighborhood of Washington as were at Petersburg and Richmond. Lee was trying to lift siege of Petersburg by shifting the seat of war to the vicinity of Washington.
The Federal advance at Second Deep Bottom on August 14 gained little ground and only captured a few seacoast howitzers at the foot off New Market Heights, but it restored the initiative to Grant. Lee recalled Hampton's division from its journey toward northern Virginia and cancelled the orders for Field's division to go.
After August 14, Grant never relinquished the initiative.
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