The imperial Roman army was the standing force deployed by the Roman Empire during the Principate era (30 BC – AD 284). Under the founder–emperor Augustus (ruled 30 BC – AD 14 ), the legions, which were formations numbering about 5,000 heavy infantry recruited from the ranks of Roman citizens only, were transformed from mixed conscript and volunteer corps soldiers serving an average of 10 years, to all-volunteer units of long-term professionals serving a standard 25-year term. (Conscription was only decreed in emergencies.) In the later 1st century, the size of a legion's First Cohort was doubled, increasing the strength of a legion to about 5,500.