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.