 On June 9, troops led by William F. "Baldy" Smith of the 18th Corps, attacked the Dimmock Line, a series of defensive breastworks constructed to protect Petersburg.General Robert E. Lee arrived with his fabled Army of Northern Virginia, and the 292-day Siege of Petersburg began.Due to botched Union leadership and arrival of Confederate General William Mahone, the Union forces suffered a disastrous defeat at the Battle of the Crater, suffering over 4,000 casualties.