in favour with the emperor, or over time perhaps multiple emperors. The practical source of an emperor's power and authority was the military. The legionaries were paid by the Imperial treasury, and swore an annual military oath of loyalty to the emperor (sacramentum). The death of an emperor led to a crucial period of uncertainty and crisis. Most emperors indicated their choice of successor, usually a close family member or adopted heir. The new emperor had to seek a swift acknowledgement of his status and authority to stabilize the political landscape. No emperor could hope to survive, much less to reign, without the allegiance and loyalty of the Praetorian Guard and of the legions. To secure their loyalty, several emperors paid the donativum, a monetary reward. In theory, the Senate was entitled to choose the new emperor, but did so mindful of acclamation by the army or Praetorians. Military After the Punic Wars, the Imperial Roman army was composed of professional soldiers who volunteered for 20 years of active duty and five as reserves. The transition to a professional military had begun during the late Republic and was one of the many profound shifts away from republicanism, under which an army of conscripts had exercised their responsibilities as citizens in defending the homeland in a campaign against a specific threat. For Imperial Rome, the military was a full-time career in itself. The Romans expanded their war machine by "organizing the communities that they conquered in Italy into a system that generated huge reservoirs of manpower for their army... Their main demand of all defeated enemies was they provide men for the Roman army every year." The primary mission of the Roman military of the early empire was to preserve the Pax Romana. The three major divisions of the military were: the garrison at Rome, which includes the Praetorian Guard, the cohortes urbanae and the vigiles, who functioned as police and firefighters; the provincial army, comprising the Roman legions and the auxiliaries provided by the provinces (auxilia); the navy. The pervasiveness of military garrisons throughout the Empire was a major influence in the process of cultural exchange and assimilation known as "Romanization," particularly in regard to politics, the economy, and religion. Knowledge of the Roman military comes from a wide range of sources: Greek and Roman literary texts; coins with military themes; papyri preserving military documents; monuments such as Trajan's Column