PARIS — If you travel to the far north of England, you'll see Hadrian's Wall still proudly standing almost 19 centuries after it was built. Of course, it's no match for the Great Wall of China, in length or in what remains of its fortifications. But it's still an impressive construction, and its message, more complex than it seems, is once again on the modern agenda.

When it was built in the year 120 AD, emperor Hadrian's goal was to proclaim Rome's greatness but also to set territorial limits on his ambitions. Why go further and conquer the rest of the lands that constitute today's Great Britain? You have to know when and where to stop before the cost of your conquests becomes too high. If only Napoleon had learned from Hadrian's example before launching the Peninsular War and his Russian campaign.

For 300 years, Hadrian's Wall kept the "northern Barbarians" away from the Romans and provided efficient protection for the Roman empire's northernmost province.

Despite its much bigger size, the Great Wall of China still conveys the same message of self-limitation. Traditionally, Chinese leaders use its very existence to deny any desire for imperial expansion on their part. With the Great Wall, China intended to protect its interests and territory from invaders, especially the Mongols, all within clearly established territorial boundaries. Why indeed build such enormous protection if your wish is to go beyond it?

In this regard, China is very different from Russia, which defines itself around its state, its army and its empire, which is preferably always expanding. China is proud of its civilization and doesn't need to grow indefinitely to exist in its own eyes. Of course, its Asian neighbors don't share this "reassuring" take on history and believe instead that the "Middle Kingdom" is not as modest and reasonable as it claims to be.

But Hadrian's Wall and the Great Wall, both the products of two formidable empires, belong nonetheless to the same sort of constructions whose functions are twofold: to protect yourself on the one hand, and to impose limits on the other.

Migrants in Hungary near the Serbian border — Photo: Gémes Sándor/SzomSzed

Very different kinds of barriers

The Berlin Wall was erected in an entirely different situation. Its goal wasn't to prevent savage hoards from entering, but to keep its most dynamic citizens and those who longed for freedom and prosperity from fleeing. For the regime of Eastern Germany, it was a way to stop the population hemorrhage that was threatening its legitimacy and survival in the short run.

We all know what happened to this wall in November 1989. It fell, a little bit by accident, the victim of a division between two Germanies that became artificial from the moment the Soviet Union no longer had the will or the means to impose it.

The wall that today separates Israelis and Palestinians is ever higher and ever longer. But will this new "Berlin Wall of the Middle East" go as far as to hermetically separate Jerusalem? Unlike the Berlin Wall, its goal isn't to prevent Palestinians from leaving, but rather to limit their capacity to launch terrorist attacks. It's the direct consequence of the Second Intifada.

But unlike Hadrian's Wall, its protective function didn't come with a vision of self-limitation. On the contrary, Israel has continued to augment, and indeed multiply its settlements in Palestinian territories. Its land nationalism, which is becoming more and more religious, doesn't conform to the rational logic of an empire of a different dimension than tiny Israel. The north of England was already so far from Rome. The West Bank is so close to Jerusalem. And in the Internet and cellphone era, the notion of space and distance are dramatically different.

There are, finally, walls like the one that divides the island of Cyprus, and whose main function is to freeze the status quo with Turkey and to prevent new violence.

Any comparison between the past and the present is of course dangerous and artificial. But the fact remains that in the face of growing chaos in the world, there is a sort of nostalgia, or a temptation to erect new walls. Is it possible to build efficient walls to protect ourselves from the influx of political refugees, like the victims of the Syrian war, or of economic migrants who want to cross from Mexico into the United States?

In other words, are there such things as wise walls and crazy walls? Walls, on the one hand, that really protect us, from ourselves and from others, and walls that achieve nothing beyond pushing problems away so we can avoid having to solve them? In a world without clear and recognized borders, how can we talk about international order if we don't know anymore who owns or controls such and such land?

When she opened Germany's gates to the waves of refugees, Chancellor Angela Merkel followed her own conscience. She deserves our respect. She must now reassure her population by proving to them that the influx of refugees will be controlled. And this isn't a matter a walls, unlike what the Hungarian government would have us believe. Walls aren't a solution to Europe's refugee problem any more than they are a solution to the new explosion of violence in Jerusalem. Beyond wise and crazy walls, the ones that really are in fashion today are the walls of fear.