It has been claimed by some that the activities of partisan ranger bands in northern and western Virginia, especially those of John S. Mosby, may have prevented a Union victory in the summer or fall of 1864. A Virginian with a penchant for violence, Mosby had been practicing law at the outbreak of the war. His assignments included: private, lst Virginia Cavalry (1861); first lieutenant, 1st Virginia Cavalry (February 1862); Captain, PACS (March 15, 1863); Major, PACS (March 26, 1863); major, 43rd Virginia Cavalry Battalion June 10, 1863); Lieutenant Colonel, 43rd Virginia Cavalry Battalion January 21, 1864); and Colonel, Mosby's (Va.) Cavalry Regiment (December 7, 1864).

Originally an enlisted man and officer in the 1st Virginia Cavalry, he came into conflict with that unit's colonel, "Grumble Jones," and joined JEB Stuart's staff as a scout. During the Peninsula Campaign he paved the way for Stuart's famous ride around McClellan. After a brief period of captivity in July 1862 he rejoined Stuart and was rewarded with the authority to raise a band of partisans for service in the Loudoun Valley in northern Virginia. Originally a battalion, his command was raised to a regiment in the last months of the war.

In the meantime he managed to wreak havoc among the Union supply lines, forcing field commanders to detach large numbers of troops to guard their communications. His forays took him within the lines guarding Washington, with Mosby himself often doing the advance scouting in disguise.

Early in 1863, with 29 men, he rode into Fairfax Court House and roused Union General Edwin H. Stoughton from bed with a slap on the rear end. Following the capture of Generals Crook and Kelley by McNeil's partisans, Mosby complimented them, stating that he would have to ride into Washington and bring out Abraham Lincoln to top their success. On another occasion he came near capturing the train on which Grant was traveling.

The disruption of supply lines and the constant disappearance of couriers frustrated army, and lesser-group, commanders to such a degree that some took to the summary execution of guerrillas, i.e. partisan rangers. George Custer executed six of Mosby's men in 1864, and the partisan chief retaliated with seven of Custer's. A note attached to one of the bodies stated that Mosby would treat all further captives as prisoners of war unless Custer committed some new act of cruelty. The killings stopped.

With the surrender of Lee, Mosby simply disbanded his command on April 20, 1865, rather than formally surrender. While the partisans were certainly a nuisance to federal commanders, it is an open question as to how effective they were in prolonging the conflict. Many Southerners were very critical of the partisans, only some Southerners excepting Mosby's command.

Not pardoned until 1866, Mosby practiced law and befriended Grant. For supporting Grant, a Republican, in the 1868 and 1872 elections, he earned the emnity of many Southerners. He received an appointment as U.S. consul in Hong Kong and other government posts.