Gaius Marius changed the Roman army in 105 B.C.E. These changes were instrumental in changing the military system. Marius took a citizen militia force and slowly transitioned it into a professional army. Crisis, political power, and military experience allowed Marius to change the military system. Marius created a modular soldier capable of completing various tasks to meet any new threat. This modular soldier system became the basis of Roman military organization for the next 300 years. This thesis argues that professional armies rather than citizen militia forces are more successful on the battlefield. Furthermore, it concludes that instituting military change is a complex problem requiring the right contributing factors for implementation. Finally, the thesis analyzes current issues facing the U.S. Army as it struggles to find a balance between the National Guard, regular forces, and preparing for the next war.