Fort Sumter was attacked by rebel forces in April 1861, starting the Civil War. Lincoln called on Virginia to stop the rebellion but it joined the Confederacy. The first major battle was at Bull Run, where the Union initially succeeded but the Confederates rallied around Stonewall Jackson and forced the Union to retreat. The Union then captured New Orleans and cut the Confederacy in half by controlling the Mississippi River, achieving key victories. The ironclad ships Monitor and Merrimack fought to a standstill in the first battle between ironclad warships. Robert E. Lee outwitted several Union generals and invaded Maryland, but the bloodiest battle so far occurred at Antietam with heavy losses on both sides.