Zosimus, New History 2.34
Zosimus (Greek Ζώσιμος): Early Byzantine, pagan author of a history of the Roman Empire, published in the first quarter of the sixth century CE.
The translation of Zosimus' New History offered here was printed in 1814 by W. Green and T. Chaplin in London, and was probably prepared by J. Davis of the Military Chronicle and Military Classics Office. The translator is anonymous. The text was found at Tertullian.org. The notes were added by Jona Lendering.
[2.34.1] Constantine likewise adopted another measure, which gave the barbarians free access into the Roman dominions. For the Roman Empire, as I have related, was, by the care of Diocletian, protected on its remote frontiers by towns and fortresses, in which soldiers were placed. It was consequently impossible for the barbarians to pass them, there being always a sufficient force to oppose their inroads.
[2.34.2] But Constantine destroyed that security by removing the greater part of the soldiers from those barriers of the frontiers, and placing them in towns that had no need of defenders,note[Zosimus misrepresents the strategy of "defense in depth".] thus depriving those who were exposed to the barbarians of all defense, and oppressing the towns that were quiet with so great a multitude of soldiers, that many of them were totally forsaken by the inhabitants. He likewise rendered his soldiers effeminate by accustoming them to public spectacles and pleasures. To speak in plain terms, he was the first cause of the affairs of the empire declining to their present miserable state.