Final answer:
A foreign soldier hired to fight for Rome was known as a 'foederati', part of the diverse military that included soldiers from allied tribes and states.
Step-by-step explanation:
The term that describes a foreign soldier hired to fight to protect Rome's borders is foederati. These were members of foreign states and tribes that had treaties with Rome, allowing them semi-autonomy in exchange for their pledge of military service to the Roman Empire. The foederati would serve alongside Roman soldiers in an increasingly diverse military, and after their service, they could integrate into Roman society. In particular, Germanic peoples often acted as foederati, aiding the Roman military efforts and settling along Rome's borders. The integration of these soldiers into Roman life was part of a larger pattern where the military served as a means to assimilate provincial subjects and promote a shared Roman identity.