They were not overexposed militarily. After conquering and integrating the whole mediterranean basin they only had two rivaling neighbors: Germanic peoples at t

They were not overexposed militarily. After conquering and integrating the whole mediterranean basin they only had two rivaling neighbors: Germanic peoples at the north and the parthians-sassanid-persians in the east. In times of integrity they managed just fine. The only reason the germans survived history to hurt the empire was because the Antonine emperors didn't want to genocide them. Then the empire had a stint of epidemics and bad luck that degradated socitey and the political system, leading to constant civil war.