What defensive measure did the Romans use to protect of decline after?

Answer 1

The decline of the Roman Empire was caused by a period of military anarchy historians call the Crisis of the Third Century. There were civil wars between military commanders who laid claim of the title of emperor. Many emperors were murdered. This also encouraged invasions of the empire by peoples close to its frontiers. The Roman army managed to repulse them, but became overstretched. There was also hyperinflation.

The Emperor Diocletian ended the anarchy. He reformed the administration of the empire in a way that decreased the power of the provinical governors, who were also military commanders and who had caused the military anarchy. He decreased the power of the governors by decreasing the size the provinces by doubling their number and by stripping them of their military role. He also strengthened the imperial bureaucracy to improve central control.

The emperor Constantine divided the army into the border army and the field army. The border army remained, as previously, stationed along the frontiers. The field army was brought further inside the empire and was supposed to intervene when the border army could stop an invasion. This redeployment made it easier fro the emperor to control the bulk of the army. Constantine also introduced a new gold coin which in the long run resolved the problem of hyperinflation. All these measures stopped the decline of the Roman Empire.