During the third and fourth centuries the Roman armies, whose legions were primarily composed of Roman citizens with auxiliary troops from subject areas within the Empire, began hiring mercenaries from outside the Empire to fill their legions. Many of these came from the Germanic tribes. I remember reading Gibbons' The History of the Decline and Fall of the Roman Empire when I was in high school and I seem to remember that the citizens, enjoying the prosperity of Empire, didn't want to join the legions (twenty five year term of service), so started paying mercenaries take their places instead.
In an interesting turn of events, the German parliament (Bundeswehr) has recently been considering recruiting foreigners to fill it's army ranks.
As Marcus Aurelius once said "almost all of the transactions in the time of Vespasian differed little from those of the present day" or to quote a more modern version from Ecclesiastes "there is nothing new under the sun".
I wonder if the results will be the same. In 410 AD Rome was sacked by the Visigoths, a milestone in the decline and fall of the Empire. We don't sack cities anymore but what might be the modern equivalent?