First Bull Run
At the outbreak of the Civil War, Burnside was a brigadier general in the Rhode Island Militia. He raised a regiment, the 1st Rhode Island Volunteer Infantry, and was appointed its colonel on May 2, 1861. Within a month, he ascended to brigade command in the Department of Northeast Virginia. He commanded the brigade without distinction at the First Battle of Bull Run in July, committing his troops piecemeal, and took over division command temporarily for wounded Brig. Gen. David Hunter. After his 90-day regiment was mustered out of service, he was promoted to brigadier general of volunteers on August 6, and was assigned to train provisional brigades in the nascent Army of the Potomac.
 North Carolina
Burnside commanded the Coast Division, or North Carolina Expeditionary Force—three brigades assembled in Annapolis, Maryland, which formed the nucleus for his future IX Corps—and the Department of North Carolina, from September 1861 until July 1862. He conducted a successful amphibious campaign that closed over 80% of the North Carolina sea coast to Confederate shipping for the remainder of the war. For his successes at the battles of Roanoke Island and New Bern, the first significant Union victories in the Eastern Theater, he was promoted to major general on March 18. In July, his forces were transported north to Newport News, Virginia, and became the IX Corps of the Army of the Potomac.
Following Maj. Gen. George B. McClellan's failure in the Peninsula Campaign, Burnside was offered command of the Army of the Potomac. Refusing this opportunity—because of his loyalty to McClellan and because he understood his own lack of military experience— he detached part of his corps in support of Maj. Gen. John Pope's Army of Virginia in the Northern Virginia Campaign. Telegrams extremely critical of Pope's abilities as a commander from Maj. Gen. Fitz John Porter that he received at this time and forwarded on to his superiors in concurrence would later play a significant role in Porter's court-martial, in which Burnside would appear as a star witness.
Burnside was given command of the "Right Wing" of the Army of the Potomac (the I Corps and IX Corps) at the start of the Maryland Campaign for the Battle of South Mountain, but McClellan separated the two corps at the Battle of Antietam, placing them on opposite ends of the Union battle line, returning Burnside to command of just the IX Corps. Implicitly refusing to give up his higher authority, Burnside treated first Maj. Gen. Jesse L. Reno (killed at South Mountain) and then Brig. Gen. Jacob D. Cox as the corps commander, funneling orders to the corps through them. This cumbersome arrangement contributed to his slowness in attacking and crossing what is now called "Burnside's Bridge" on the southern flank of the Union line.
Burnside did not perform adequate reconnaissance of the area, and instead of taking advantage of several easy fording sites out of range of the enemy, his troops were forced into repeated assaults across the narrow bridge which was dominated by Confederate sharpshooters on high ground. By noon, McClellan was losing patience. He sent a succession of couriers to motivate Burnside to move forward. He ordered one aide, "Tell him if it costs 10,000 men he must go now." He increased the pressure by sending his inspector general to confront Burnside, who reacted indignantly: "McClellan appears to think I am not trying my best to carry this bridge; you are the third or fourth one who has been to me this morning with similar orders." The delay allowed Maj. Gen. A.P. Hill's Confederate division to come up from Harpers Ferry and repulse the Union breakthrough. McClellan refused Burnside's requests for reinforcements, and the battle ended in a tactical stalemate.
Main article: Battle of Fredericksburg Union General Ambrose Burnside, 1862.
McClellan was removed after failing to pursue General Robert E. Lee's retreat from Antietam, and Burnside was assigned to command the Army of the Potomac on November 7, 1862. He reluctantly obeyed this order, the third such in his brief career. President Abraham Lincoln pressured Burnside to take aggressive action and on November 14 approved his plan to capture the Confederate capital at Richmond, Virginia. This plan led to a humiliating and costly Union defeat at the Battle of Fredericksburg on December 13. His advance upon Fredericksburg was rapid, but planning in marshaling pontoon bridges for crossing the Rappahannock River and his own reluctance to deploy portions of his army across fording points later delayed the attack. This allowed Gen. Lee to concentrate along Marye's Heights just west of town and easily repulse the Union attacks. Assaults south of town, which were supposed to be the main avenue of attack, were also mismanaged, and initial Union breakthroughs went unsupported. Upset by the failure of his plan and by the enormous casualties of his repeated, futile frontal assaults, Burnside declared that he would lead an assault by his old corps. His corps commanders talked him out of it, but relations between the commander and his subordinates were strained. Accepting full blame, he offered to retire from the U.S. Army, but this was refused.
In January 1863, Burnside launched a second offensive against Lee, but it bogged down in winter rains before it accomplished anything and has been derisively called the Mud March. In its wake, he asked that several officers, who were openly insubordinate, be relieved of duty and court-martialed; he also offered to resign. Lincoln chose the latter option on January 26 and replaced him with Maj. Gen. Joseph Hooker, one of the officers who had conspired against Burnside.
 East Tennessee
Lincoln was unwilling to lose Burnside from the Army and assigned him to command the Department of the Ohio and his old IX Corps. In Ohio, Burnside issued his controversial General Order Number 38, making it a crime to express any kind of opposition to the war. Burnside used it to arrest former Ohio congressman and candidate for governor of Ohio, Clement Vallandigham, a prominent leader in the copperhead peace movement, and try him in a military court (despite the fact that he was a civilian). Burnside also dealt with Confederate raiders such as John Hunt Morgan.
In the Knoxville Campaign, Burnside advanced to Knoxville, Tennessee, first bypassing the Confederate-held Cumberland Gap. After occupying Knoxville unopposed, he sent troops back to the Cumberland Gap. Brig. Gen. John W. Frazer, the Confederate commander, refused to surrender in the face of two Union brigades and Burnside arrived with a third, forcing the surrender of Frazer and 2,300 Confederates. After Union Maj. Gen. William S. Rosecrans was defeated at the Battle of Chickamauga, Burnside was pursued by Lt. Gen. James Longstreet, against whose troops he had battled at Marye's Heights. Burnside skillfully outmaneuvered Longstreet at the Battle of Campbell's Station and was able to reach his entrenchments and safety in Knoxville, where he was briefly besieged until the Confederate defeat at the Battle of Fort Sanders outside the city. Tying down Longstreet's corps at Knoxville contributed to Gen. Braxton Bragg's defeat by Maj. Gen. Ulysses S. Grant at Chattanooga. Troops under Maj. Gen. William T. Sherman marched to Burnside's aid, but the siege had already been lifted; Longstreet withdrew, eventually returning to Virginia.
 Overland Campaign
Burnside was ordered to take the IX Corps back to the Eastern Theater, where, in Annapolis, Maryland, he built it up to a strength of over 21,000 effectives. The IX Corps fought in the Overland Campaign of May 1864 as an independent command, reporting initially to Grant; his corps was not assigned to the Army of the Potomac because Burnside outranked its commander, Maj. Gen. George G. Meade, who had been a division commander under Burnside at Fredericksburg. This cumbersome arrangement was rectified on May 24 just before the Battle of North Anna, when Burnside agreed to waive his precedence of rank and was placed under Meade's direct command.
Burnside fought at the battles of Wilderness and Spotsylvania Court House, where he did not perform in a distinguished manner, attacking piecemeal and appearing reluctant to commit his troops to the frontal assaults that characterized these battles. After North Anna and Cold Harbor, he took his place in the siege lines at Petersburg.
 The Crater
As the two armies faced the stalemate of trench warfare at Petersburg in July 1864, Burnside agreed to a plan suggested by a regiment of Pennsylvania coal miners in his corps: dig a mine under a fort in the Confederate entrenchments and ignite explosives there to achieve a surprise breakthrough. The fort was destroyed on July 30 in what is known as the Battle of the Crater. Because of interference from Meade, Burnside was ordered, only hours before the infantry attack, not to use his division of black troops, which had been specially trained for this mission. He was forced to use untrained white troops instead. He could not decide which division to choose as a replacement, so he had his three subordinate commanders draw lots. The division chosen by chance was that commanded by Brig. Gen. James H. Ledlie, who failed to brief the men on what was expected of them and was reported during the battle to be drunk well behind the lines, providing no leadership. Ledlie's men entered the huge crater instead of going around it, becoming trapped, and were subjected to murderous fire from Confederates around the rim, resulting in high casualties.
Burnside was relieved of command on August 14 and sent on leave by Grant; Meade never recalled him to duty. A court of inquiry later placed the blame for the Crater fiasco on Burnside and his subordinates. In December, Burnside met with President Lincoln and General Grant about his future. He was contemplating resignation, but Lincoln and Grant requested that he remain in the Army. At the end of the interview, Burnside wrote, "I was not informed of any duty upon which I am to be placed." He finally resigned his commission on April 15, 1865.