Eastern Theater of the American Civil War 1861–1863: Because of the fierce resistance of a few initial Confederate forces at Manassas, Virginia, in July 1861, a march by Union troops under the command of Maj. Gen. Irvin McDowell on the Confederate forces there was halted in the First Battle of Bull Run, or First Manassas,^[100] 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.

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.

Maj. Gen. George B. McClellan took command of the Union Army of the Potomac on July 26 (he was briefly general-in-chief of all the Union armies, but was subsequently relieved of that post in favor of Maj. Gen. Henry W. Halleck), and the war began in earnest in 1862. Upon the strong urging of President Lincoln to begin offensive operations, McClellan attacked Virginia in the spring of 1862 by way of the peninsula between the York River and James River, southeast of Richmond. Although McClellan's army reached the gates of Richmond in the Peninsula Campaign, Johnston halted his advance at the Battle of Seven Pines, then General Robert E. Lee and top subordinates James Longstreet and Stonewall Jackson defeated McClellan in the Seven Days Battles and forced his retreat. The Northern Virginia Campaign, which included the Second Battle of Bull Run, ended in yet another victory for the South. McClellan resisted General-in-Chief Halleck's orders to send reinforcements to John Pope's Union Army of Virginia, which made it easier for Lee's Confederates to defeat twice the number of combined enemy troops.

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. 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.

When the cautious McClellan failed to follow up on Antietam, he was replaced by Maj. Gen. Ambrose Burnside. Burnside was soon defeated at the Battle of Fredericksburg on December 13, 1862, when over twelve thousand Union soldiers were killed or wounded during repeated futile frontal assaults against Marye's Heights. After the battle, Burnside was replaced by Maj. Gen. Joseph Hooker. Hooker, too, proved unable to defeat Lee's army; despite outnumbering the Confederates by more than two to one, he was humiliated in the Battle of Chancellorsville in May 1863. He was replaced by Maj. Gen. George Meade during Lee's second invasion of the North, in June. Meade defeated Lee at the Battle of Gettysburg^[111] (July 1 to July 3, 1863). This was the bloodiest battle of the war, and has been called the war's turning point. Pickett's Charge on July 3 is often considered the high-water mark of the Confederacy because it signaled the collapse of serious Confederate threats of victory. Lee's army suffered 28,000 casualties (versus Meade's 23,000). However, Lincoln was angry that Meade failed to intercept Lee's retreat, and after Meade's inconclusive fall campaign, Lincoln turned to the Western Theater for new leadership. At the same time the Confederate stronghold of Vicksburg surrendered, giving the Union control of the Mississippi River, permanently isolating the western Confederacy, and producing the new leader Lincoln needed, Ulysses S. Grant.