Don Carlos Buell (March 23, 1818 – November 19, 1898) was a United States Army officer who fought in the Seminole War, the Mexican–American War, and the American Civil War. Buell led Union armies in two major Civil War battles—Shiloh and Perryville. The nation was angered at his failure to defeat the outnumbered Confederate Army after Perryville, or to secure East Tennessee. Historians generally concur that he was a brave and industrious master of logistics, but was too cautious and rigid to meet the challenges presented in 1862. Buell was relieved of field command in late 1862 and made no more significant military contributions before his resignation in 1864.

Don Carlos Buell was born in Lowell, Ohio, the eldest of nine children born to Salmon and Elizabeth Buell. He was a first cousin of George P. Buell, also a Union general. Buell's father died when he was 8 years old, and his uncle took him in and raised him. As a child, Buell had a difficult time making friends due to his distant, introverted personality and was often made fun of by other children. After winning a fight with a neighborhood bully, he became awakened to the idea that discipline and determination could overcome any obstacle.^{[citation needed]} Buell's uncle sent him to a Presbyterian school which stressed duty, self-discipline, patriotism, and belief in a Supreme Being.

George Buell obtained for his nephew an appointment to West Point, but despite his high intelligence and good math skills, he accumulated numerous demerits and disciplinary problems and graduated in 1841 32nd in his class of 52. After graduation, Buell was commissioned a second lieutenant in the 3rd U.S. Infantry regiment and sent to fight in the Seminole Wars in Florida but did not see any combat. After the 3rd Infantry was transferred to Illinois, Buell found himself court-martialed for getting into an argument with an enlisted man and beating him over the head with the blunt end of his sword. However, an Army tribunal cleared him of any wrongdoing. There was considerable opposition to the verdict, and even General Winfield Scott felt that Buell needed to be punished for his actions, but the court would not retry the case.

In the Mexican–American War, he served under both Zachary Taylor and Winfield Scott. He was brevetted three times for bravery and was wounded at Churubusco. Between the wars he served in the U.S. Army Adjutant General's office and as an adjutant in California, reaching the rank of captain in 1851 and lieutenant colonel by the time the Civil War began.

At the start of the Civil War, Buell sought an important command, but instead his friend George McClellan emerged as the champion of the Union war effort. Buell himself was sent all the way out to California. After the Union defeat at Bull Run, McClellan summoned him back east where he was quickly promoted to brigadier general of volunteers, to rank from May 17, 1861. Buell received offers to take a command in Kentucky, but instead he stayed in Washington helping organize the nascent Army of the Potomac and being appointed as a division commander. In November, McClellan succeeded Winfield Scott as general-in-chief of the Army, and decided to post Buell out west, dividing the trans-Appalachia theater between him and Maj. Gen Henry Halleck. In November 1861, he succeeded Brig. Gen. William T. Sherman in command at Louisville, Kentucky, as commander of the newly formed Army of the Ohio, which at this time was a barely-disciplined rabble. Buell immediately set himself to work shaping the raw recruits into a fighting force. Although the Lincoln administration pressured Buell to occupy Eastern Tennessee, an area of strong Unionist sentiment, Buell was in no hurry, and even McClellan became impatient with his slow progress. Buell's excuse was that the railroad network in this area was poor, and he would have to rely on wagons for army supply that could be vulnerable to Confederate cavalry. Instead, he proposed a coordinated effort between him and Halleck to cut off Nashville. Halleck reluctantly agreed to the plan, which was helped along by Grant's capture of Fort Henry and Fort Donelson. Although the city fell to the Army of the Ohio on February 25, 1862, Halleck's relationship with Buell was strained. The same month, Andrew Johnson was made military governor of Tennessee and developed a lasting grudge against Buell for failing to liberate Eastern Tennessee. On March 21, Buell was promoted to major general of volunteers, but at the same time, Halleck rose to department commander which made Buell subordinate to him.

At the start of April, Buell was ordered to reinforce Grant's Army of the Tennessee, then encamped at Pittsburg Landing next to the Tennessee River. On the morning of April 6, the Confederates launched a surprise attack on Grant's army, beginning one of the largest and bloodiest battles of the war. After the Army of the Ohio arrived the next day, the combined Union forces repulsed the Confederates. Although Buell was the junior of the two generals in rank, he insisted that he was acting independently and would not accept orders from Grant. Buell considered himself the victor of Shiloh and denigrated Grant's contribution, writing after the war that he had no "marked influence that he exerted upon the fortune of the day." Contemporary historians, such as Larry Daniels and Kenneth W. Noe, consider that Grant actually saved himself by the conclusion of the first day of battle and that the rivalry between Grant and Buell hampered the conduct of battle on the second day. The commanders operated almost completely independently of each other and Buell "proved slow and hesitant to commit himself."

Following Shiloh, Governor Johnson objected to Buell's plans to withdraw the Nashville garrison on the grounds that Confederate sympathy in the city was still strong. However, Halleck sided with Buell and insisted that all available troops in the department were needed for the assault on Corinth. Henry Halleck arrived in person to take command of Grant and Buell's armies. The combined Union force, 100,000 men strong, began an extremely sluggish pursuit against P.G.T. Beauregard's Army of Mississippi, which had retreated into northern Mississippi. Despite a more than 2-1 numerical advantage, Halleck moved slowly. However, Buell was even slower and quickly caught the ire of Halleck. During the march to Corinth, Buell took extensive pauses to repair the railroad lines and would not entertain suggestions of allowing his army to live off the land. To that end, he court-martialed Col. John Turchin for allowing his soldiers to loot area homes. This action was not popular either with Buell's troops or the War Department, and President Lincoln ultimately overturned the verdict, while Turchin eventually got promoted to brigadier general.

The Siege of Corinth ended in the Confederates abandoning the city on May 25. Afterwards, Halleck split up the two armies and sent Buell eastward to capture Chattanooga while Grant remained in the Corinth area. In July, Halleck was summoned back to Washington to replace George McClellan as commander-in-chief of all Union armies, thus effectively returning the two Western armies to independent action. Buell's advance towards Chattanooga nearly rivaled the earlier march on Corinth for sluggishness, with extensive pauses to stop and repair railroad lines. When Nathan Bedford Forrest's cavalry then ransacked the Army of Ohio's supply lines, Buell all but terminated the effort to take Chattanooga. Thus after a busy winter and spring, activity in the Western theater during the summer of 1862 almost completely ground to a halt. On July 17, Buell was promoted to colonel in the regular army.