Belloc begins by describing "the barbarian" invasions of Rome, but quickly moves from a cause to describing the barbarian invasions as a symptom of internal rot. For, "if the fundamental institutions of a polity are no longer regarded as fundamental by its citizens, that polity is about to pass through the total change which in a living organism we call death." These, I think, are Belloc's Barbarians - those who willingly take the advantages of many who have come before them, but are unwilling or incapable of sustaining them.
Belloc confirms this, in an oft-quoted (ofter quoted than, perhaps, the chapter is read):
The Barbarian hopes — and that is the mark of him, that he can have his cake and eat it too. He will consume what civilization has slowly produced after generations of selection and effort, but he will not be at pains to replace such goods, nor indeed has he a comprehension of the virtue that has brought them into being. Discipline seems to him irrational, on which account he is ever marvelling that civilization should have offended him with priests and soldiers.... In a word, the Barbarian is discoverable everywhere in this, that he cannot make: that he can befog and destroy but that he cannot sustain; and of every Barbarian in the decline or peril of every civilization exactly that has been true.
Not necessarily external to the society, the Barbarian, but perhaps growing among the populace, like weeds. Belloc observes that the barbarian is a sort of barometer in society - if the barbarian succeeds in changing the fundamentals of society, in becoming respected and listened to, then the society is fractured and undergoing "total change."
In our own society, consider the (now nearly successful) crusade to both legalize and normalize gay marriage. As legal challenges to "normal" marriage mounted, gay marriage's proponents engaged in a simultaneous cultural campaign to normalize gay marriage as "the same thing" as heterosexual marriage. Belloc is here, too, noting in his time that both proponents of eased divorce laws and opponents used the same utilitarian concepts to justify their positions, rarely (if ever) using the language of "sacrament."
Marriage, then, is one of the fundamental things in a culture, in our culture, that is no longer regarded as such by many. Equally true and growing foreign to most of our culture is the idea that marriage is tied to child-bearing and child-rearing. We see also that, with taboos against gay marriage falling, other taboos have as well - against bearing children outside of marriage, against voyeurism (porn), against divorce.
Now we turn to consider a candidate for president who has had multiple wives, as well as admitting to affairs publicly. This does not seem to have hurt his chances in any way - indeed, it is barely a topic of discussion. That he made a great deal of money in a business formerly considered at least problematic (casinos) is never mentioned. This is a man who recently (though since recanted) stated that he would deliberately target families of terrorists, calmly stated that he has no problem with waterboarding, and declared that he was adequately endowed - all during a presidential debate. He is the very figure of a barbarian, even having declared that, if his daughter were not his daughter, he would be dating her.
And, unfortunately, he is entirely a product of modern American consumer culture. It is not that divorces, adultery, gambling, and other such have not existed for generations - it is that now they are not discussed as problematic, and even considered "normal." (The incest taboo, thankfully, still exists, but even here and there there are signs that this is weakening.) If this were not the case, Mr. Trump could not consistently garner 30% or more of votes in primaries - his supporters must be ignorant of these things, or must not care about them.
And so, we should not look for an external enemy for America. Our foreign wars may even be a distraction for us, perceiving an enemy abroad when we grow them quickly at home. If Belloc (and others like him) are at all accurate, we are facing a time of turmoil, perhaps war. On a final cautionary note from Belloc, as I recall that Trump was a reality TV personality for several years:
In a word, the Barbarian is discoverable everywhere in this that he cannot make; that he can befog or destroy, but that he cannot sus tain; and of every Barbarian in the decline or peril of every civilisation exactly that has been true.
We sit by and watch the Barbarian, we tolerate him ; in the long stretches of peace we are not afraid.
We are tickled by his irreverence, his comic inversion of our old certitudes and our fixed creeds refreshes us : we laugh. But as we laugh we are watched by large and awful faces from beyond : and on these faces there is no smile.
We permit our jaded intellects to play with drugs of novelty for the fresh sensation they arouse, though we know well there is no good in them, but only wasting at the last.