At the start of the Civil War in 1861, Captain Pleasonton traveled with the 2nd Dragoons from Fort Crittenden, Utah Territory, to Washington D.C. Despite active politicking on his part, attempting to capitalize on the faded political connections of his father (who had died in 1855), Pleasonton did not earn the rapid promotions of some of his colleagues.
He was transferred to the 2nd U.S. Cavalry Regiment on August 3, 1861.
He was promoted to major on February 15, 1862.
He fought without incident or prominence in the Peninsula Campaign, providing Army of the Potomac commander Major General George McClellan with little accurate or valuable information.
On May 1, 1862, Pleasonton was nominated by President Abraham Lincoln for promotion to the grade of brigadier general of volunteers to rank from July 16, 1862, and the United States Senate confirmed the appointment on July 16, 1862.
President Lincoln formally appointed Pleasonton to the grade on July 18, 1862.
He then commanded a brigade of cavalry in the Army of the Potomac.

On September 2, 1862, Pleasonton assumed division command in the cavalry.
He was wounded in the right ear by the concussion of an artillery shell at the Battle of Antietam on September 17.
Ever ambitious, Pleasonton was displeased that he was not promoted to major general of volunteers for his actions, claiming erroneously that his division, and particularly the horse artillery assigned to him, had had a decisive effect on the battle.
He did receive a brevet promotion to lieutenant colonel in the regular army, probably based solely on the inflated claims of his battle report, which were not substantiated by the reports of other generals.

At the Battle of Chancellorsville, Pleasonton continued his practice of self-promotion.
He claimed that he temporarily halted an attack by Stonewall Jackson's Corps and that he was able to prevent the total destruction of the Union XI Corps on May 2, 1863.
He was persuasive enough that the commander of the Army of the Potomac, Maj.
Gen. Joseph Hooker, told President Abraham Lincoln that Pleasonton "saved the Union Army" at Chancellorsville.
Battle reports, however, indicate that Pleasonton's role was considerably less important than he claimed, involving only a small detachment of Confederate infantry on Hazel Grove.
Nevertheless, his claims earned him an appointment to the grade of major general of volunteers on and to rank from June 22, 1863, and when the inept Cavalry Corps commander, Maj.
Gen. George Stoneman, was relieved after Chancellorsville, Hooker named Pleasonton as his temporary replacement.
Pleasonton could not accept even this elevated role gracefully.
He wrote to Gen. Hooker "I cannot...remain silent as to the unsatisfactory condition in which I find this corps...the responsibility of its present state...does not belong to me."
At the start of the Civil War in 1861, Captain Pleasonton traveled with the 2nd Dragoons from Fort Crittenden, Utah Territory, to Washington D.C. Despite active politicking on his part, attempting to capitalize on the faded political connections of his father (who had died in 1855), Pleasonton did not earn the rapid promotions of some of his colleagues. He was transferred to the 2nd U.S. Cavalry Regiment on August 3, 1861. He was promoted to major on February 15, 1862. He fought without incident or prominence in the Peninsula Campaign, providing Army of the Potomac commander Major General George McClellan with little accurate or valuable information. On May 1, 1862, Pleasonton was nominated by President Abraham Lincoln for promotion to the grade of brigadier general of volunteers to rank from July 16, 1862, and the United States Senate confirmed the appointment on July 16, 1862. President Lincoln formally appointed Pleasonton to the grade on July 18, 1862. He then commanded a brigade of cavalry in the Army of the Potomac.
On September 2, 1862, Pleasonton assumed division command in the cavalry. He was wounded in the right ear by the concussion of an artillery shell at the Battle of Antietam on September 17. Ever ambitious, Pleasonton was displeased that he was not promoted to major general of volunteers for his actions, claiming erroneously that his division, and particularly the horse artillery assigned to him, had had a decisive effect on the battle. He did receive a brevet promotion to lieutenant colonel in the regular army, probably based solely on the inflated claims of his battle report, which were not substantiated by the reports of other generals.
At the Battle of Chancellorsville, Pleasonton continued his practice of self-promotion. He claimed that he temporarily halted an attack by Stonewall Jackson's Corps and that he was able to prevent the total destruction of the Union XI Corps on May 2, 1863. He was persuasive enough that the commander of the Army of the Potomac, Maj. Gen. Joseph Hooker, told President Abraham Lincoln that Pleasonton "saved the Union Army" at Chancellorsville. Battle reports, however, indicate that Pleasonton's role was considerably less important than he claimed, involving only a small detachment of Confederate infantry on Hazel Grove. Nevertheless, his claims earned him an appointment to the grade of major general of volunteers on and to rank from June 22, 1863, and when the inept Cavalry Corps commander, Maj. Gen. George Stoneman, was relieved after Chancellorsville, Hooker named Pleasonton as his temporary replacement. Pleasonton could not accept even this elevated role gracefully. He wrote to Gen. Hooker "I cannot...remain silent as to the unsatisfactory condition in which I find this corps...the responsibility of its present state...does not belong to me."
Alfred Pleasonton