Undetected by Lee, Grant moved his army south of the James River, freed Butler from the Bermuda Hundred, and advanced toward Petersburg, Virginia's central railroad hub. Beauregard defended Petersburg, and Lee's veteran reinforcements arrived on June 18, resulting in a nine-month siege. Northern resentment grew. Sheridan was assigned command of the Union Army of the Shenandoah and Grant directed him to "follow the enemy to their death" in the Shenandoah Valley. When Sheridan suffered attacks by John S. Mosby's irregular Confederate cavalry, Grant recommended rounding up their families for imprisonment at Fort McHenry. After Grant's abortive attempt to capture Petersburg, Lincoln supported Grant in his decision to continue and visited Grant's headquarters at City Point on June 21 to assess the state of the army and meet with Grant and Admiral Porter. By the time Lincoln departed his appreciation for Grant had grown.