"The Roman military kept the empire's borders secure" is true of the century following the Pax Romana.
Explanation:
Plague and annexations ravaged the empire at late 3rd century CE, and cracks were starting to emerge. Following Marcus Aurelius' demise in 180 CE and the emergence of his heir Emperor Commodus, after nearly two hundred years, the principle of Pax Romana became an absolute necessity.
The Roman Empire's boundaries used troops, forts, walls and natural barriers to defend frontiers. The word "Pax Romana," meaning literally "Roman stability," refers to the era from 27 B.C.E. until 180 C.E. In Roman Imperialism. This period of 200 years witnessed unparalleled peace and economic wellness across the Empire.
