Why the Roman Empire Fell.

 This chart by Dr. A.J. Parker shows the number of Mediterranean shipwrecks by date of sinking.  The number of wrecks is proportional to the number of sailings.  And the number of ship sailings is a measure of economic activity, especially so in an age when everything moved by water. 
  Most interesting is the dating.  Notice the steady, almost exponential grow that starts 500-600 BC and keeps growing strongly until the first century BC.  About then, the Roman Republic came unglued and the Roman emperors, Julius, Augustus and company take over.  Notice also the decline in shipping that sets in at about the same time.  By the time we get to Constantine, commerce is down by two thirds from its peak.  In short, Constantine, who starts the retrenchment of the Empire, had only a third of the wealth to pay his armies and support his government that Augustus had at his disposal. 
   With this chart one can make a good argument that the freer and more democratic institutions of the Republic encouraged commerce and industry, whereas the policies of the emperors was hostile. It is generally agreed that the Empire fell when it was no longer able to pay the professional Roman Army that had kept the barbarians out for hundreds of years.