What it was
The legion was the main unit of the Roman army, a formation of several thousand heavy infantry citizens with attached cavalry and support, the backbone of Rome’s military power.
Role
Disciplined and drilled, organized into cohorts and centuries, legions conquered and held the empire and also built roads, forts, and frontiers, spreading Roman ways. Reforms associated with Marius (late 2nd century BC) opened recruitment and professionalized the army, and legions became power bases for ambitious generals.
Fate
The legions evolved over centuries and declined with the Western Empire, increasingly relying on non-Roman troops. Roman military organization and engineering left a lasting mark on the art of war.