Roman engineers built bridges and roads, which allows for more quicker travel from one part of the empire to another. They also built forts and walled cities in many places, and filled them with supplies, so that if there was a revolt, the Romans there (if they are outnumbered) are able to take refuge inside until reinforcements arrived.
These all help their military have a higher chance of surviving, faster transportation, etc, and it made their armies the most strongest in that era
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