Today's operational environment finds irregular warfare alive and well. Irregular forces' organization, size, composition, and depth of support vary greatly on the battlefield. These dynamics correspondingly impact their overall influence and effect on a given conflict, as irregulars may serve as a mere harassment or weigh heavily on the final results. The state of Missouri and the surrounding region were no strangers to guerrilla and irregular warfare during the American Civil War. Pre-existing conflict and competing sentiments for secession or neutrality laid the roots for a violent struggle to determine the fate of Missouri as a Union or Confederate state. Pro-Confederate Missourians formed irregular organizations to fight for a secessionist Missouri. These forces associated with the Confederate government and army to pursue common objectives; however, the relationships between these organizations were largely ill-defined. Two predominant pro-Confederate organizations, the Missouri State Guard (MSG) and William Quantrills band of guerrillas, approached irregular warfare in and around Missouri with very different methods and tactics. The operational histories of the MSG and Quantrills band reveal instances of coordinated operations with the conventional army/ government and unilateral activities alike. Through a detailed study of these forces and the larger operational context for the Confederacy, this monograph finds that there was no consistent, cohesive relationship between regular and irregular forces, and that this lack of structure impaired operational planning and effectiveness. The paper concludes by looking at the relevance to the contemporary operating environment and offers considerations and recommendations for todays operational planner.