Several cavalry raids were organized, which inflicted severe injuries upon the enemy's communications south of Petersburg. The celebrated mine was sprung on the 30th of July, which blew up one of the most important of the rebel forts, involving a battery and the greater part of a regiment in its destruction; but the result, which had promised so well, realized nothing but disaster. As soon as Lee discovered that Hunter was retreating through West Virginia, he sent Jubal Early, with a picked force of twenty-five thousand men, down the Shenandoah Valley, to threaten Washington, and to capture it, if practicable, hoping thus to distract the attention of Grant, and cause him to relax the "anaconda" gripe in which he held the rebel army. This army swept fiercely down the valley, and driving the small Union force in the vicinity before it, crossed the Potomac. Strong bodies of cavalry, under Mosby, rushed through Maryland, plundering Hagerstown and Frederick City, robbing the stores, and extorting money from the people to save their houses from being burned. They destroyed a portion of the Baltimore and Ohio Railroad, and threatened Baltimore and Washington. General Wallace gathered a force of eight thousand men, and attempted to dispute the passage of Early's army; but as the enemy were three to his one, he was compelled to fall back, though he fought a sharp battle before doing so. Washington and Baltimore were now greatly alarmed, and the citizens were called to arms. The enemy came within five miles of the capital. Grant sent the Sixth Corps, under Wright, and a portion of the Nineteenth, which had just arrived from New Orleans, for its protection. There was some heavy skirmishing near the capital, but the rebels soon retired. Wright was ordered to follow them; and, having overtaken Early, a smart engagement ensued, in which the enemy was defeated. The Shenandoah region gave the lieutenant general a great deal of trouble. He found that Early had no intention of returning to Richmond, but had established himself in the valley; was gathering the rich harvests there, and sending large supplies to the rebel capital. He visited Hunter in person, and gave him particular instructions to follow Early, and to destroy all supplies; but finding Hunter willing to be relieved, he soon after assigned Sheridan to the Middle Military Division, which included all this section, and all the troops in Washington and its vicinity. The bold cavalryman was not only the most dashing officer in the army, but one of the best and most skilful generals. He soon brought order and harmony out of the confusion and complications which had so disturbed the general-in-chief. Grant cautioned him at first to avoid a general engagement, fearful, in case of defeat, of exposing Maryland, Pennsylvania, and the capital to new incursions.