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.[251] 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.[252][w] 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,[253] 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.[254] Grant admitted that the overall mining tactic had been a "stupendous failure".