At the outbreak of the Civil War in April 1861, Buell was transferred to California to serve as adjutant of the Department of the Pacific. He was then recalled to Washington, D.C., after only a few months and commissioned a brigadier general of volunteers. Buell spent the next several months training new recruits in General George B. McClellan’s Army of the Potomac.
In November 1861 Buell was sent to the war’s Western Theater in Kentucky and placed in command of the Army of the Ohio. He was given orders from President Abraham Lincoln and General George B. McClellan to invade eastern Tennessee, an area believed to harbor sympathy for the Union cause. Buell—citing a lack of reliable transport for his over 50,000-strong army—instead elected to move on Nashville. He was able to claim the city with minimal effort in February 1862, and was promoted to major general shortly thereafter.
In March 1862 General Henry Halleck ordered Buell to move south to rendezvous with General Ulysses S. Grant at Pittsburg Landing, Tennessee. Before the operation could be coordinated, Confederate General Albert Sidney Johnston ambushed Grant’s forces at the Battle of Shiloh on April 6, 1862. Buell arrived that same evening with nearly 20,000 of his men. His added reinforcements helped ensure a Union victory on the battle’s second day, when a massive Union counterattack routed forces commanded by General P.G.T. Beauregard. Buell would later insist that he deserved credit for turning the tide at Shiloh, while others—in particular Ulysses S. Grant and William T. Sherman—argued that his troops ultimately had little effect on the outcome.
After the Battle of Shiloh Buell’s army continued with General Henry Halleck to the vital rail junction at Corinth, Mississippi, where they besieged P.G.T. Beauregard and forced him to abandon the city to Union control. Buell’s Army of the Ohio was subsequently detached from the main Union force and sent toward Chattanooga, Tennessee. Buell was ordered to repair railroad supply lines during his advance, but his operations were hampered by raids from Confederate cavalry under the command of Nathan Bedford Forrest and John Hunt Morgan. In August 1862 Morgan succeeded in cutting off Buell’s communication and supply lines and slowing his advance on Chattanooga.
During this time Buell came under fire for what many saw as his sympathy toward the civilian population in the South. Not only was he hesitant to wage the kind of total war later employed by General William T. Sherman, Buell court-martialed pillagers and was also reluctant to harbor escaped slaves in his camps. This drew the ire of many in the North, and Buell was nearly relieved of duty under intense political pressure.