I agree that Rome built her roads for her military, and her military benefited from them.
However, I am not sure that Rome mobilised an unusually large proportion of their population. For the longest time, Romans were better at warfare than most of their neighbours, but I'm not sure they actually did more of it.
Their neighbours weren't as organised, and not as organised on as large a scale; but they perhaps mobilised just as much?
"So the Romans have something on the order of 750,000 adult males liable for conscription, of which some 526,000 are in the band the Romans generally think of as ‘fighting age’ (17-46). That is a staggering figure, though of course the Romans never put that many men under arms at one time: peak roman mobilizations in 212 and 211 are around 185,000 (which is still more than double peak attested Seleucid or Ptolemaic mobilizations)."
However, I am not sure that Rome mobilised an unusually large proportion of their population. For the longest time, Romans were better at warfare than most of their neighbours, but I'm not sure they actually did more of it.
Their neighbours weren't as organised, and not as organised on as large a scale; but they perhaps mobilised just as much?