On April 10, 1861, Brig. Gen. Beauregard, in command of the provisional Confederate forces at Charleston, South Carolina, demanded the surrender of the Union garrison of Fort Sumter in Charleston Harbor. Garrison commander Anderson refused. On April 12, Confederate batteries opened fire on the fort, which was unable to reply effectively. At 2:30 pm, April 13, Major Anderson surrendered Fort Sumter, evacuating the garrison on the following day. The bombardment of Fort Sumter was the opening engagement of the American Civil War. Although there were no casualties during the bombardment, one Union artillerist was killed and three wounded (one mortally) when a cannon exploded prematurely while firing a salute during the evacuation on April 14.

This was the first major land battle of the armies in Virginia. On July 16, 1861, the untried Union army under Brig. Gen. Irvin McDowell marched from Washington against the Confederate army, which was drawn up behind Bull Run beyond Centreville. On the 21st, McDowell crossed at Sudley Ford and attacked the Confederate left flank on Matthews Hill. Fighting raged throughout the day as Confederate forces were driven back to Henry Hill. Late in the afternoon, Confederate reinforcements (one brigade arriving by rail from the Shenandoah Valley) extended and broke the Union right flank. The Federal retreat rapidly deteriorated into a rout. Although victorious, Confederate forces were too disorganized to pursue. Confederate Gen. Bee and Col. Bartow were killed. Thomas J. Jackson earned the nom de guerre "Stonewall." By July 22, the shattered Union army reached the safety of Washington. This battle convinced the Lincoln administration that the war would be a long and costly affair. McDowell was relieved of command of the Union army and replaced by Maj. Gen. George B. McClellan, who set about reorganizing and training the troops.

As a result of the fall of Forts Henry and Donelson, Confederate Gen. Albert Sidney Johnston, the commander in the area, was forced to fall back, giving up Kentucky and much of West and Middle Tennessee. He chose Corinth, Mississippi, a major transportation center, as the staging area for an offensive against Maj. Gen. Ulysses S. Grant and his Army of the Tennessee before the Army of the Ohio, under Maj. Gen. Don Carlos Buell, could join it. The Confederate retrenchment was a surprise, although a pleasant one, to the Union forces, and it took Grant, with about 40,000 men, some time to mount a southern offensive, along the Tennessee River, toward Pittsburg Landing. Grant received orders to await Buell’s Army of the Ohio at Pittsburg Landing. Grant did not choose to fortify his position; rather, he set about drilling his men many of which were raw recruits. Johnston originally planned to attack Grant on April 4, but delays postponed it until the 6th. Attacking the Union troops on the morning of the 6th, the Confederates surprised them, routing many. Some Federals made determined stands and by afternoon, they had established a battle line at the sunken road, known as the "Hornets Nest." Repeated Rebel attacks failed to carry the Hornets Nest, but massed artillery helped to turn the tide as Confederates surrounded the Union troops and captured, killed, or wounded most. Johnston had been mortally wounded earlier and his second in command, Gen. P.G.T. Beauregard, took over. The Union troops established another line covering Pittsburg Landing, anchored with artillery and augmented by Buell’s men who began to arrive and take up positions. Fighting continued until after dark, but the Federals held. By the next morning, the combined Federal forces numbered about 40,000, outnumbering Beauregard’s army of less than 30,000. Beauregard was unaware of the arrival of Buell’s army and launched a counterattack in response to a two-mile advance by William Nelson’s division of Buell’s army at 6:00 am, which was, at first, successful. Union troops stiffened and began forcing the Confederates back. Beauregard ordered a counterattack, which stopped the Union advance but did not break its battle line. At this point, Beauregard realized that he could not win and, having suffered too many casualties, he retired from the field and headed back to Corinth. On the 8th, Grant sent Brig. Gen. William T. Sherman, with two brigades, and Brig. Gen. Thomas J. Wood, with his division, in pursuit of Beauregard. They ran into the Rebel rearguard, commanded by Col. Nathan Bedford Forrest, at Fallen Timbers. Forrest’s aggressive tactics, although eventually contained, influenced the Union troops to return to Pittsburg Landing. Grant’s mastery of the Confederate forces continued; he had beaten them once again. The Confederates continued to fall back until launching their mid-August offensive.

In order to draw Pope’s army into battle, Jackson ordered an attack on a Federal column that was passing across his front on the Warrenton Turnpike on August 28, 1862. The fighting at Brawner Farm lasted several hours and resulted in a stalemate. Pope became convinced that he had trapped Jackson and concentrated the bulk of his army against him. On August 29, Pope launched a series of assaults against Jackson’s position along an unfinished railroad grade. The attacks were repulsed with heavy casualties on both sides. At noon, Longstreet arrived on the field from Thoroughfare Gap and took position on Jackson’s right flank. On August 30, Pope renewed his attacks, seemingly unaware that Longstreet was on the field. When massed Confederate artillery devastated a Union assault by Fitz John Porter’s command, Longstreet’s wing of 28,000 men counterattacked in the largest, simultaneous mass assault of the war. The Union left flank was crushed and the army driven back to Bull Run. Only an effective Union rearguard action prevented a replay of the First Manassas disaster. Pope’s retreat to Centreville was precipitous, nonetheless. The next day, Lee ordered his army in pursuit. This was the decisive battle of the Northern Virginia Campaign.

On September 16, 1862 Maj. Gen. George B. McClellan confronted Lee’s Army of Northern Virginia at Sharpsburg, Maryland. At dawn September 17, Hooker’s corps mounted a powerful assault on Lee’s left flank that began the single bloodiest day in American military history. Attacks and counterattacks swept across Miller’s cornfield and fighting swirled around the Dunker Church. Union assaults against the Sunken Road eventually pierced the Confederate center, but the Federal advantage was not followed up. Late in the day, Burnside’s corps finally got into action, crossing the stone bridge over Antietam Creek and rolling up the Confederate right. At a crucial moment, A.P. Hill’s division arrived from Harpers Ferry and counterattacked, driving back Burnside and saving the day. Although outnumbered two-to-one, Lee committed his entire force, while McClellan sent in less than three-quarters of his army, enabling Lee to fight the Federals to a standstill. During the night, both armies consolidated their lines. In spite of crippling casualties, Lee continued to skirmish with McClellan throughout the 18th, while removing his wounded south of the river. McClellan did not renew the assaults. After dark, Lee ordered the battered Army of Northern Virginia to withdraw across the Potomac into the Shenandoah Valley.

On November 14, 1862 Burnside, now in command of the Army of the Potomac, sent a corps to occupy the vicinity of Falmouth near Fredericksburg. The rest of the army soon followed. Lee reacted by entrenching his army on the heights behind the town. On December 11, Union engineers laid five pontoon bridges across the Rappahannock under fire. On the 12th, the Federal army crossed over, and on December 13, Burnside mounted a series of futile frontal assaults on Prospect Hill and Marye’s Heights that resulted in staggering casualties. Meade’s division, on the Union left flank, briefly penetrated Jackson’s line but was driven back by a counterattack. Union generals C. Feger Jackson and George Bayard, and Confederate generals Thomas R.R. Cobb and Maxey Gregg were killed. On December 15, Burnside called off the offensive and recrossed the river, ending the campaign. Burnside initiated a new offensive in January 1863, which quickly bogged down in the winter mud. The abortive "Mud March" and other failures led to Burnside’s replacement by Maj. Gen. Joseph Hooker in January 1863.

On April 27, Maj. Gen. Joseph Hooker led the V, IX, and XII Corps on a campaign to turn the Confederate left flank by crossing the Rappahannock and Rapidan Rivers above Fredericksburg. Passing the Rapidan via Germanna and Ely’s Fords, the Federals concentrated near Chancellorsville on April 30 and May 1. The III Corps was ordered to join the army via United States Ford. Sedgwick’s VI Corps and Gibbon’s division remained to demonstrate against the Confederates at Fredericksburg. In the meantime, Lee left a covering force under Maj. Gen. Jubal Early in Fredericksburg and marched with the rest of the army to confront the Federals. As Hooker’s army moved toward Fredericksburg on the Orange Turnpike, they encountered increasing Confederate resistance. Hearing reports of overwhelming Confederate force, Hooker ordered his army to suspend the advance and to concentrate again at Chancellorsville. Pressed closely by Lee’s advance, Hooker adopted a defensive posture, thus giving Lee the initiative. On the morning of May 2, Lt. Gen. T.J. Jackson directed his corps on a march against the Federal left flank, which was reported to be "hanging in the air." Fighting was sporadic on other portions of the field throughout the day, as Jackson’s column reached its jump-off point. At 5:20 pm, Jackson’s line surged forward in an overwhelming attack that crushed the Union XI Corps. Federal troops rallied, resisted the advance, and counterattacked. Disorganization on both sides and darkness ended the fighting. While making a night reconnaissance, Jackson was mortally wounded by his own men and carried from the field. J.E.B. Stuart took temporary command of Jackson’s Corps. On May 3, the Confederates attacked with both wings of the army and massed their artillery at Hazel Grove. This finally broke the Federal line at Chancellorsville. Hooker withdrew a mile and entrenched in a defensive "U" with his back to the river at United States Ford. Union generals Berry and Whipple and Confederate general Paxton were killed; Stonewall Jackson was mortally wounded. On the night of May 5-6, after Union reverses at Salem Church, Hooker recrossed to the north bank of the Rappahannock. This battle was considered by many historians to be Lee’s greatest victory.

In May and June of 1863, Maj. Gen. Ulysses S. Grant’s armies converged on Vicksburg, investing the city and entrapping a Confederate army under Lt. Gen. John Pemberton. On July 4, Vicksburg surrendered after prolonged siege operations. This was the culmination of one of the most brilliant military campaigns of the war. With the loss of Pemberton’s army and this vital stronghold on the Mississippi, the Confederacy was effectively split in half. Grant's successes in the West boosted his reputation, leading ultimately to his appointment as General-in-Chief of the Union armies.

Gen. Robert E. Lee concentrated his full strength against Maj. Gen. George G. Meade’s Army of the Potomac at the crossroads county seat of Gettysburg. On July 1, Confederate forces converged on the town from west and north, driving Union defenders back through the streets to Cemetery Hill. During the night, reinforcements arrived for both sides. On July 2, Lee attempted to envelop the Federals, first striking the Union left flank at the Peach Orchard, Wheatfield, Devil’s Den, and the Round Tops with Longstreet’s and Hill’s divisions, and then attacking the Union right at Culp’s and East Cemetery Hills with Ewell’s divisions. By evening, the Federals retained Little Round Top and had repulsed most of Ewell’s men. During the morning of July 3, the Confederate infantry were driven from their last toe-hold on Culp’s Hill. In the afternoon, after a preliminary artillery bombardment, Lee attacked the Union center on Cemetery Ridge. The Pickett-Pettigrew assault (more popularly, Pickett’s Charge) momentarily pierced the Union line but was driven back with severe casualties. Stuart’s cavalry attempted to gain the Union rear but was repulsed. On July 4, Lee began withdrawing his army toward Williamsport on the Potomac River. His train of wounded stretched more than fourteen miles.

After the Tullahoma Campaign, Rosecrans renewed his offensive, aiming to force the Confederates out of Chattanooga. The three army corps comprising Rosecrans’ s army split and set out for Chattanooga by separate routes. In early September 1863, Rosecrans consolidated his forces scattered in Tennessee and Georgia and forced Bragg’s army out of Chattanooga, heading south. The Union troops followed it and brushed with it at Davis’ Cross Roads. Bragg was determined to reoccupy Chattanooga and decided to meet a part of Rosecrans’s army, defeat them, and then move back into the city. On the 17th he headed north, intending to meet and beat the XXI Army Corps. As Bragg marched north on the 18th, his cavalry and infantry fought with Union cavalry and mounted infantry which were armed with Spencer repeating rifles. Fighting began in earnest on the morning of the 19th, and Bragg’s men hammered but did not break the Union line. The next day, Bragg continued his assault on the Union line on the left, and in late morning, Rosecrans was informed that he had a gap in his line. In moving units to shore up the supposed gap, Rosencrans created one, and James Longstreet’s men promptly exploited it, driving one-third of the Union army, including Rosecrans himself, from the field. George H. Thomas took over command and began consolidating forces on Horseshoe Ridge and Snodgrass Hill. Although the Rebels launched determined assaults on these forces, they held until after dark. Thomas then led these men from the field leaving it to the Confederates. The Union retired to Chattanooga while the Rebels occupied the surrounding heights.

On May 31, 1864 Sheridan’s cavalry seized the vital crossroads of Old Cold Harbor. Early on June 1, relying heavily on their new repeating carbines and shallow entrenchments, Sheridan’s troopers threw back an attack by Confederate infantry. Confederate reinforcements arrived from Richmond and from the Totopotomoy Creek lines. Late on June 1, the Union VI and XVIII Corps reached Cold Harbor and assaulted the Confederate works with some success. By June 2, both armies were on the field, forming on a seven-mile front that extended from Bethesda Church to the Chickahominy River. At dawn June 3, the II and XVIII Corps, followed later by the IX Corps, assaulted along the Bethesda Church-Cold Harbor line and were slaughtered at all points. Grant commented in his memoirs that this was the only attack he wished he had never ordered. The armies confronted each other on these lines until the night of June 12, when Grant again advanced by his left flank, marching to James River. On June 14, the II Corps was ferried across the river at Wilcox’s Landing by transports. On June 15, the rest of the army began crossing on a 2,200-foot long pontoon bridge at Weyanoke. Abandoning the well-defended approaches to Richmond, Grant sought to shift his army quickly south of the river to threaten Petersburg.