Fort Monroe, the largest stone fortification ever built in the United States, was begun in 1819 and took 15 years to complete. Second Lt. Robert E. Lee served there as an engineer from 1831 to 1834. During the Civil War, its position near the entrance to the Chesapeake Bay, the James and York rivers, and the southern tip of the James/York Peninsula made it strategically important to the Union as the only Union-controlled fortified base in the Upper South. The North Atlantic Blockading Squadron used it as a base, and several amphibious expeditions to capture Confederate ports were organized there. It also became known as the "Freedom Fort" by blacks who sought escape from bondage after Brig. Gen. Benjamin F. Butler, while commanding there in May 1861, declared escaping slaves "contraband of war," a first step toward the war becoming a war about freedom. Maj. Gen. George B. McClellan used it as his primary base during the first phase of the Peninsula Campaign, and President Abraham Lincoln came there in May 1862 to encourage cooperation between the army and navy and speed the campaign. While there, Lincoln formulated plans for the capture of Norfolk, which would lead to the eventual destruction of the Confederate ironclad CSS Virginia (Merrimack). Fort Monroe remained an important Union base throughout the war, including as the launching point for the war's last army-navy amphibious operation against Fort Fisher in North Carolina. Lincoln returned in February 1865 for the Hampton Roads Peace Conference, held aboard a steamer in Hampton Roads harbor, but the meeting failed to restore peace. After the war, former Confederate president Jefferson Davis was imprisoned there.