there was halted in the First Battle of Bull Run , or First Manassas . [ 168 ] McDowell's troops were forced back to Washington, D.C., by the Confederates under the command of Generals Joseph E. Johnston and P. G. T. Beauregard . It was in this battle that Confederate General Thomas Jackson received the nickname of "Stonewall" because he stood like a stone wall against Union troops. [ 169 ] Confederate ironclads at Norfolk and New Orleans dispersed blockade, until Union ironclads could defeat them Alarmed at the loss, and in an attempt to prevent more slave states from leaving the Union, the U.S. Congress passed the Crittenden-Johnson Resolution on July 25 of that year, which stated that the war was being fought to preserve the Union and not to end slavery. Emboldened by Second Bull Run, the Confederacy made its first invasion of the North. General Lee led 45,000 men of the Army of Northern Virginia across the Potomac River into Maryland on September 5. Lincoln then restored Pope's troops to McClellan. McClellan and Lee fought at the Battle of Antietam near Sharpsburg , Maryland, on September 17, 1862, the bloodiest single day in United States military history. [ 173 ] [ 175 ] Lee's army, checked at last, returned to Virginia before McClellan could destroy it. Antietam is considered a Union victory because it halted Lee's invasion of the North and provided an opportunity for Lincoln to announce his Emancipation Proclamation . [ 176 ] When the