Alfred Pleasonton (1824-1897) was born in Washington, D.C., the son of Stephen who was well known at the time of Alfred's birth. During the War of 1812, as a U.S. State Department employee, Stephen's personal initiative saved crucial documents in the National Archives from destruction by the British invaders of Washington, including the original Declaration of Independence and the U.S. Constitution. Alfred graduated from the U.S. Military Academy at West Point in 1844 and was commissioned a second lieutenant in the 1st U.S. Dragoons (heavy cavalry), stationed first at Fort Atkinson, Iowa. He accompanied his unit for frontier duty in Minnesota, Iowa, and Texas. With the 2nd Dragoons, he fought in the Mexican-American War and received a brevet promotion to first lieutenant for gallantry in the Battle of Palo Alto and the Battle of Resaca de la Palma, Texas, in 1846. He served as regimental adjutant after the war and was promoted to captain in 1855.

At the start of the Civil War, Captain Pleasonton traveled with the 2nd Dragoons from Fort Crittenden, Utah Territory, to Washington D.C. He fought in the Peninsula Campaign and was promoted to Brigadier General on July 16th, 1862, commanding a brigade of cavalry in the Army of the Potomac. On September 2nd, Pleasonton assumed division command in the cavalry and was wounded by an artillery shell at the Battle of Antietam. At the Battle of Chancellorsville, Pleasanton 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 2nd, 1863. He was persuasive enough that the commander of the Army of the Potomac, Maj. Gen. Joseph Hooker, told President Abraham Lincoln that Pleasanton "saved the Union Army" at Chancellorsville. Battle reports, however, indicate that Pleasanton's role was not quite what he claimed, involving only a small detachment of Confederate infantry on Hazel Grove. Nevertheless, he a promotion to Major General of Volunteers as of June 22nd, 1863, and when the inept Cavalry Corps commander, Maj. Gen. George Stoneman, was relieved after Chancellorsville, Hooker named Pleasanton as his temporary replacement. Pleasonton 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."

Pleasonton's first combat in his new role was in the Gettysburg Campaign. He led Union cavalry forces in the Battle of Brandy Station, the largest predominantly cavalry battle of the war. The Union cavalry essentially stumbled into J.E.B. Stuart's Confederate cavalry and the 14-hour battle was bloody yet inconclusive, although Stuart was taken by surprise and the Union horsemen had a newfound confidence in their abilities. Some officers criticized Pleasonton for not aggressively defeating Stuart at Brandy Station. Gen. Hooker had ordered Pleasonton to "disperse and destroy" the Confederate cavalry near Culpeper, Virginia, but Pleasonton claimed that he had only been ordered to make a "reconnaissance in force toward Culpeper", thus rationalizing his actions. Later, the Army of Northern Virginia, under Gen. Robert E. Lee, was able to slip past Union forces through the Shenandoah Valley and north into Pennsylvania. During this period, Pleasonton exercised political influence by promoting the son of a U.S. Congressman, Captain Elon J. Farnsworth, a member of his staff, directly to Brigadier General. Pleasonton also promoted Captain Wesley Merritt and First Lieutenant George Armstrong Custer to Brigadier General. Pleasonton corresponded with the congressman and complained about his lack of men and horses in comparison to Jeb Stuart's; he also politicked to acquire the cavalry forces of Maj. Gen. Julius Stahel, who commanded the cavalry in the defenses of Washington. His persuasion proved effective. Stahel was relieved of his command and his troops were reassigned to Pleasonton, which enraged Hooker.

In the Battle of Gettysburg, Pleasanton's new commander, Maj. Gen. George G. Meade, kept Pleasonton on a short leash. For the three days of the battle, Pleasonton was forced to remain with Meade at army headquarters, rather than with the Cavalry Corps headquarters nearby. In postwar writings, Pleasonton attempted to portray his role in the battle as being a major one, including predicting to Meade that the town of Gettysburg would be the decisive point and, after the Confederate defeat in Pickett's Charge, that he urged Meade to attack Gen. Lee and finish him off. Pleasonton cannot be blamed for the unfortunate cavalry action on July 3rd, when Meade ordered the division of Brig. Gen. Judson Kilpatrick to attack the right flank of the Confederate army, which resulted in a suicidal assault against entrenched infantry and the futile death of Elon Farnsworth.

Pleasonton was then transferred to the Trans-Mississippi Theater and commanded the District of Central Missouri and the District of St. Louis in 1864. He performed exceptionally well and defeated Gen. Sterling Price at Westport, the Battle of Byram's Ford and Marais des Cygnes, effectively ending the last Confederate threat in the West. He received a brevet promotion to Brigadier General in the regular Army for the campaign in Missouri, and to Major General for his overall conduct in the war, both as of March 13th, 1865.