The retreat of Johnston's army up the Peninsula uncovered the Confederate navy base at Norfolk, Virginia. The Confederate installations there were burnt and abandoned. The most important ship the Confederates had for the defense of Richmond, the CSS Virginia or Merrimack, was threatened by this movements. For many weeks her presence had kept the Federals on the edge, since she had shown that she could destroy any of the United States Navy's wooden ships with no problems. The North had the Monitor, but since the two ships had tied when they had fought each other, the result of a rematch was doubtful. Flag Officer Josiah Tattnall, commander of the Virginia, had been unwilling to risk an attack because of the doubtfulness of the Virginia's engines. With the abandonment of Norfolk, he had to either fight, try to sail up river, or destroy the ship. He was unwilling to fight, and so he made preparations to sail up river. There were sandbars, and so he lightened the ship as much as possible. However, this uncovered places on the ship that were usually below the water line and were not covered by armor. This meant that if the ship was stuck on a bar, she would be vulnerable to Union fire. Tattnall was unwilling to take the risk to sail up river, so he gave orders early on the morning of May 11th, 150 years ago today, to abandon the ship and set her alight. When the fires reached the magazine she exploded with a terrific blast, and thus the Confederacy's most famous ironclad was destroyed.