From a biography by a student named Anderson http://xroads.virginia.edu/~CLASS/am483_97/projects/anderson/bio.html

When Virginia followed other Southern states and voted to secede from the Union following Abraham Lincoln's election to the presidency in 1860, Mosby decided to enlist in the Confederate army. At first Mosby followed a local company of infantry, but quickly transfered to the cavalry corps of Gen. J.E.B. Stuart, and became acquainted with the duties of a scout. Before too long, however, Mosby became anxious to form his own command, that would not be bound by traditional army conventions. In January 1863, Stuart approved Mosby's plan and gave him a few men to begin his operation. Mosby and his partisan rangers were later incorporated into the regular Confederate army. Their primary objective consisted of destroying railroad supply lines between Washington and Northern Virginia, as well as intercepting dispatches and horses and capturing Union soldiers. Mosby's numbers rose from one dozen to a few hundred by the end of the war. Mosby's rank likewise rose steadily; his final promotion to colonel came in January 1865. Gen. Robert E. Lee cited Mosby for meritorious service more often than any other Confederate officer during the course of the war.