Despite their many successes, Roman leaders continually struggled with indiscipline in their own ranks as they battled Rome's opponents. Desertion and defection were steps that soldiers often undertook to avoid their obligated service. Previous scholarship has largely overlooked this aspect of Roman warfare. This dissertation analyzes why Roman soldiers began turning to desertion and defection throughout the Republican period. Such cases were generally rare in early Rome, but the expanding responsibilities and hardships of warfare in the Middle Republic caused them to rise, as did the sizeable growth of the Roman community. The civil wars of the Late Republic saw especially high cases of such acts, as generals incentivized defections in their opponents ranks. Roman desertion was not unique, but a common occurrence in ancient warfare. This dissertation also addresses how Romans capitalized on desertion and defection in warfare. The Second Punic War offers an example of how Rome achieved victory by encouraging defection in its enemy's alliances. Romans also relied heavily on defectors as a source of intelligence and as a tool in siege warfare. The moral forces of commitment, discipline, dissatisfaction, and desertion were often as important as the tactics and technologies of the participants in Rome's wars.