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 colored 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".