"..... the buffer zone has disappeared. The geographical transition from civilization to barbarism is now no longer gradual but it is abrupt. To use the appropriate Latin words, which bring out both the kinship and the contrast between the two types of contact, a limen, or threshold, which was a zone, has been replaced by a limes or military frontier. Across this line, a baffled dominant minority and an unconquered external proletariat now face one another under arms; and this military front is a bar to the passage of all social radiation except that of military technique - an article of exchange which makes for war and not for peace between those who give and take it. ..... the cardinal fact (is) that this temporary and precarious balance of forces inevitably tilts, with the passage of time, in favour of the Barbarians." From Arnold Toynbee, ""A study of History." (image above from Wikipedia)
The more I look at what's happening in the world today, the more it seems to me that we are following an already beaten path: we are going along the same way that the Roman Empire followed a couple of millennia ago, only faster. For every event that takes place today, you can find a parallel event that led the old Empire to its end. So, the present refugee crisis in Europe has a parallel in the crisis of the 1st century AD that led the empire to lock itself inside a set of fortifications. Is that the destiny that Europe faces today? Are we going to build a wall to keep the invaders out? A look of what happened to the Romans may tell us something about this point.
You probably know the story, told by Suetonius, of Emperor Augustus walking at night in his palace, asking the dead general who had led the legions at Teutoburg, "give me back my legions". But that was only a symptom of a general fear that the Barbarians would soon march all the way to Rome.
Fear often leads to overreacting, and there is no doubt that the Romans overreacted. It would take four centuries after Teutoburg before Rome was besieged and taken by a Barbarian army. But, at that time, fear reigned in the Empire. A few years afterward, the Romans invaded Germany again with no less than 8 legions; without accomplishing much more than demonstrating that there was no way for them to conquer and subdue the Germans. Then, they changed their strategy: if the barbarians couldn't be defeated, they could at least be kept out of the empire.
Border fortifications around the Roman Empire had been existing even before the battle of Teutoburg, but, after the battle, they were greatly expanded and strengthened. The final result was the system of border fortifications around the Empire that we call today "limes" (even though the Romans didn't use that word). A series of walls that started at the Northern border of Britannia and circled the whole Empire, even though not continuously.
Were the fortifications useful? For one thing, it is true that they kept the Barbarian armies at bay for a few centuries. But it is also true that they must have been atrociously expensive. So much that, eventually, the economy of the Roman Empire became engaged in only two activities: cultivating grain and maintaining the border fortifications. Unfortunately, we lack the data we would need in order to quantify these expenses, but I think it can be proposed that the border fortifications were such an economic burden that they were a major factor leading to the eventual demise of the Empire.
The fortifications had another problem; perhaps even bigger: in the effort of keeping the Barbarians out, the Romans had locked themselves in a no-win situation. They badly needed slaves for their agriculture and soldiers for their armies, and this manpower, during the late years of the Empire, would largely come from Barbarian people. But how would Barbarians came in if the borders of the Empire were closed? The wall, in principle, should have kept Barbarian armies out, but let Barbarian workers in. However, the Romans never could convince the Barbarians that it was a good idea for them to come into the Empire to become slaves in the name of the free market. It is possible that the walls were not only too expensive for their purpose, but even counterproductive as they kept out manpower that the Empire desperately needed.
Over time, it became impossible for the Empire to maintain the fortifications and, with the beginning of the 5th century, they were abandoned. According to Gibbon, it was in the winter of 406 AD that the frozen Rhine made it possible for a large number of Barbarians to cross the river and to march into the Empire unopposed in a crisis that resembles very much the flow of refugees pouring into Europe today. A few years later, in 410 AD, Rome was sacked for the first time in the Imperial age by the Visigoths. Then, in 455 AD, Rome was sacked again by the Vandals and, this time, it was truly the end of the Western Roman Empire. For a few decades afterward, some individuals still claimed the title of "Roman Emperor"; but nobody was paying much attention to them. The walls had not helped Rome to survive.
Is this what's going to happen to Europe in our times? Are we going to make the same mistake the Romans made and ruin ourselves by building an expensive wall to stop invaders from entering
Right now, walls don't seem to be in the plans, also because European didn't experience an equivalent of the defeat of Teutoburg (yet). So, it seems that the European governments are seeing the refugees from abroad as cheap manpower that Europe desperately needs - just like Rome did, long ago. But it is also clear that the situation can't remain the way it is for a long time, with millions of refugees pushing at the borders of Europe, chased away from their lands by a combination of wars and climate change. At some point, someone will start panicking and call for a defensive wall.
Modern Europe has seen already a wall separating it in two halves, the one that was called "the Iron Curtain". Also that wall didn't bring good luck to those who built it, whose economy collapsed among other factors also under the weight of maintaining the walls. A new wall to keep North Africans and Middle Eastern people out would probably cripple Europe forever. Will it be built? We can't say, right now, but one thing that we learn from history is that we never learn from history