But of all the problems Europe faces, it is the migration crisis that could become existential. In the Middle East, North Africa, and the region stretching from the Sahel to the Horn of Africa, there are about 20 million displaced people; civil wars, widespread violence, and failed states are becoming the norm. If Europe has trouble absorbing a million refugees, how will it eventually handle 20 million? Unless Europe can defend its external borders, the Schengen agreement will collapse and internal borders will return, ending freedom of movement – a key principle of European integration – within most of the EU. But the solution proposed by some – close the gates to refugees – would merely worsen the problem, by destabilizing countries like Turkey, Lebanon, and Jordan, which have already absorbed millions. And paying off Turkey and others to keep the refugees would be both costly and unsustainable.

And the problems of the greater Middle East (including Afghanistan and Pakistan) and Africa cannot be resolved by military and diplomatic means alone. The economic factors driving these (and other) conflicts will worsen: global climate change is accelerating desertification and depleting water resources, with disastrous effects on agriculture and other economic activity that then trigger violence across ethnic, religious, social, and other cleavages. Nothing short of a massive, Marshall Plan-style outlay of financial resources, especially to rebuild the Middle East, will ensure long-term stability. Will Europe be able and willing to pay its share of it?

If economic solutions aren’t found, eventually these regions’ conflicts will destabilize Europe, as millions more desperate, hopeless people eventually become radicalized and blame the West for their misery. Even with an unlikely wall around Europe, many would find a way in – and some would terrorize Europe for decades to come. That’s why some commentators, inflaming the tensions, speak of barbarians at the gates and compare Europe’s situation to the beginning of the end of the Roman Empire.