Which is more dangerous: Invading Immigrants, or Large Centralized Government?

This article at the Brussel's Journal applies this question to the fall of the Roman Empire, examining two books on the topic:

What Caused Rome’s Collapse: Immigration or Centralisation?

It's a long article, great if you like history. An argument is made that invading immigrants often wanted to uphold the better parts of the Empire and it's culture, and that the true collapse of Rome came from it's large, centralized government.

After examining two detailed books about the fall of Rome, which this article is about, the author seems to lean towards Centralized Government as the greater danger. In the last paragraph he sums up this lesson as a warning for the European Union and the United States:

[...] An ex-KGB officer, Igor Panarin, has apparently argued in a recent monograph that the United States will soon split up, as the Soviet Union did twenty years ago. That spontaneous disintegration of the Stalinist Empire was the best fate that could have overcome the superannuated Bolshevik experiment and its subject peoples. In one of history’s ironies, the European nation-states began their march into lock-step rule by apparatchiks at exactly the moment when their old enemy ceased to exist. The United States, too, under Bush I and Bush II and now Obama, has embraced a new doctrine of centripetal authoritarianism and coercive ideological reconstruction. The much-to-be-hoped-for failures both of the European Community and the socialist-in-fact-but-not-by-name Democrat-Party regime in the United States, followed by the genuine re-federalization of Europe and North America, might be the most providential turn of events as the world lurches stupidly into its Twenty-First Century “Globalist” delusions.

For those who enjoy detailed historical examination, treat yourself to the whole of this article by Thomas F. Bertonneau.