After a century or so of trial and error, intermittent improvements by talented commanders, and ultimately a massively OCD organization initiated and/or reformed by Augustus, the Roman army *was* the Roman Empire. They kept a standing army that (after Augustus) rarely exceeded 280,000 (half of whom were auxiliaries) and yet Roman rule stretched the length of North Africa, and from lowland Scotland to the Persian Gulf during the reign of Trajan, and maintaining (and expanding) that with relatively few troops probably is some kind of record.
Romans used paid mercenaries to maintain the last few centuries. And I believe they paid Attila to stay away for a few years. The remarkable thing as you say was one city doing this for so long. But they had a network of help