Immigration and assimilation...
"5) Immigration Only Works As Long As There Is Assimilation: Like the United States, the Romans were great believers in bringing other people under the banners. Instead of subjugating and enslaving the foreign nations they captured, the Romans usually made them part of their alliance. In some cases they gave them full citizenship, but even a lesser form of citizenship was no small thing given the power of the Romans and how they could have behaved given their strength. This was a big part of why the Romans became so successful. However, as time went on, things began to change. The Romans had at times allowed conquered tribes to enter their territory, but they always split them up so they'd have no choice other than to live surrounded by Romans and learn their ways. As the Empire became more decadent, they allowed tribes of barbarians to move into Roman territory as a group under their old rulers. Under those circumstances, the barbarians kept their old culture. Eventually, the Romans started bringing these foreigners into their military. The Western half of the Roman Empire was especially guilty of this. In time, the foot soldiers in the military were largely German Goths who ironically were primarily needed to fight other German Goths who surged across the borders. This was problematic for a number of reasons, but just to name one, it sometimes led to Roman cities being besieged and sacked by their own unassimilated German troops. Eventually, the Western half of the Roman Empire fell as the foreign troops decided they'd rather be ruled by a barbarian king named Odoacer. Here in America, not only do we have surging immigration, we no longer encourage immigrants to assimilate to our culture. It has gotten so bad that we'd rather tell people to press 2 for Spanish instead of insisting that they learn English. Immigration has gotten so out-of-control that even suggesting that we should limit immigration or only allowing immigrants who obey our laws to come here is called "racist." The Romans eventually lost their nation over that sort of attitude and one day, the same thing may happen to us if we're not careful."
...Jim Thompson