Realizing that Washington was left unprotected due to Grant's siege of Petersburg, Lee detached a corps under the command of Lieutenant General Jubal A. Early, hoping it would force the Union army to send forces to pursue him. If Early could capture Washington, the Civil War would not have ended, but the Confederates would have embarrassed the Union and deeply wounded northern morale. Early, with 15,000 seasoned troops, marched north "down" the Shenandoah Valley, defeated Union Major General Lew Wallace at the Monocacy, and following the Battle of Fort Stevens, he reached the outskirts of Washington, causing great alarm. At Lincoln's urging, Grant dispatched the veteran Union VI Corps and parts of the XIX Corps, led by Major General Horatio Wright, to the capital. With the Union XXII Corps manning Washington's fortifications, Early was unable to take the city. The Confederate Army's mere presence close to the capital was still an embarrassment. At Petersburg, Grant blew up a section of Lee's trenches with gunpowder planted inside a huge mine tunnel dug by Pennsylvanian coal miners. The explosion dug a huge crater and opened a big gap in the Confederate line. The Union assault that followed, however, was slow and chaotic, with troops milling around inside the Crater. This allowed Lee to repulse the breakthrough.