Quantrill's Raiders were a loosely organized force of pro-Confederate bushwhackers who fought in the American Civil War under the leadership of William Clarke Quantrill. The name "Quantrill's Raiders" seems to have been attached to them long after the war, when the veterans would hold reunions.
Missouri was fertile ground for the outbreak of guerrilla warfare in late 1861. Secessionists had already been organized to some extent by the proslavery "Border Ruffian" movement of the 1850s, in which Missourians crossed the border into the Kansas Territory in an effort to make it a slave state. Unionists were less well organized, but the populace was nevertheless deeply divided.
In 1861, the campaign between Union and Missouri forces rolled back and forth across the southern half of the state, until finally the governor, Claiborne F. Jackson, and the Missouri State Guard, under the command of General Sterling Price, were largely forced into Arkansas before the end of the year. Across the countryside, however, skirmishes erupted between Unionist and secessionist Missourians, and between secessionists and Union irregulars from Kansas who entered the state to plunder.
The insurgency flared in those areas where Union forces were weakest. As Union soldiers concentrated to fight against Price's State Guard and regular Confederate forces under General Ben McCulloch, few were available to occupy the territory to the rear. It was only in late 1861, as garrisons were established in important towns, that the weaker and more poorly organized Confederate guerrillas were defeated, and stronger, more capable units came together. The most notorious of these was that led by William Clarke Quantrill.
Quantrill was not the only Confederate guerrilla operating in Missouri, but he rapidly won the greatest renown. He and his men ambushed Union patrols and supply convoys, seized the mail, and occasionally struck at undefended towns on either side of the Kansas-Missouri border. Reflecting the internecine nature of the guerrilla conflict in Missouri, Quantrill directed much of his effort against Unionist civilians, attempting to drive them from of the territory where he operated. Under his direction, Confederate partisans also perfected military tactics such as coordinated and synchronized attacks, planned dispersal after an attack using pre-planned routes and relays of horses, and other technical methods, including the use of the long-barreled revolvers that later became the preferred firearm of western lawmen and outlaws alike. The James-Younger Gang, many of whose members had ridden with Quantrill, applied these same techniques after the war.
Quantrill claimed sanction under the Confederate Partisan Ranger Act, which authorized certain guerrilla activities, and apparently he had received a regular Confederate commission as a captain. However, like almost all of the Missouri bushwhackers, he operated outside of the Confederate chain of command. Some of his activities, most notably the massacre of some 200 men and boys, as young as seven years old, in Lawrence, Kansas in August 1863, appalled the Confederate authorities. In the winter of 1862-63, when Quantrill led his men behind Confederate lines into Texas, their often lawless presence proved an embarrassment to the Confederate command. Yet the generals appreciated his effectiveness against Union forces, which never gained the upper hand over Quantrill.
Quantrill died at the hands of Union forces in Kentucky in May 1865, but his legacy would live on. Many of his men, including Frank James, rode in 1864 under one of his former lieutenants, "Bloody Bill" Anderson, who was killed in October 1864. Much of that group remained together under the leadership of Archie Clement, who kept the gang together after the war, and harassed the Republican state government of Missouri during the tumultuous year of 1866. In December 1866, state militiamen killed Clement in Lexington, Missouri, but his men continued on as outlaws, emerging in time as the James-Younger Gang.