Army of the Shenandoah and Grant directed him to "follow the enemy to their death" in the Shenandoah Valley. When Sheridan suffered attacks by John S. Mosby's irregular Confederate cavalry, Grant recommended rounding up their families for imprisonment at Fort McHenry. After Grant's abortive attempt to capture Petersburg, Lincoln supported Grant in his decision to continue and visited Grant's headquarters at City Point on June 21 to assess the state of the army and meet with Grant and Admiral Porter. By the time Lincoln departed his appreciation for Grant had grown. To strike at Lee in a timely capacity Grant was forced to use what resources were immediately available, and they were diminishing by the day. Grant had to commit badly needed troops to check Confederate General Jubal Early's raids in the Shenandoah Valley and who was getting dangerously close to the Potomac River, and Washington. By late July, at Petersburg, Grant reluctantly approved a plan to blow up part of the enemy trenches from a tunnel filled with many tons of gunpowder. The massive explosion created a crater, 170 feet across and 30 feet deep, killing an entire Confederate regiment in an instant. The poorly led Union troops under Major General Burnside and Brigadier General Ledlie, rather than encircling the crater, rushed forward and poured directly into it, which was widely deemed a mistake. Recovering from the surprise, Confederates, led by Major General William Mahone, surrounded the crater and easily picked off Union troops within it. The Union's 3,500 casualties outnumbered the Confederates' by three-to-one. The battle marked the first time that Union black troops, who endured a large proportion of the casualties, engaged in any major battle in the east. Grant admitted that the overall mining tactic had been a "stupendous failure". Grant would later meet with Lincoln and testify at a court of inquiry against Generals Burnside and Ledlie for their incompetence. In his memoirs he blamed both of them for that disastrous Union defeat. Rather than fight Lee in a full-frontal attack as he had done at Cold Harbor, Grant continued to force Lee to extend his defenses south and west of Petersburg, better allowing him to capture essential railroad links. Union forces soon captured Mobile Bay and Atlanta and now controlled the Shenandoah Valley, ensuring Lincoln's reelection in November. Sherman convinced Grant and Lincoln to send his army to march on Savannah. Sherman cut a