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When standing today at Hadrian's Wall in northern England, everything appears indistinguishably affluent and serene on both sides.

It was not nearly as calm some 1,900 years ago. In A.D. 122, the exasperated Roman emperor Hadrian ordered the construction of an 80-mile, 20-foot-high wall to protect Roman civilization in Britain from the Scottish tribes to the north.

We moderns often laugh at walls and fortified boundaries, dismissing them as hopelessly retrograde, ineffective or unnecessary. Yet they still seem to fulfill their mission on the Israeli border, the 38th parallel in Korea and the Saudi-Iraqi boundary, separating disparate states.

On the Roman side of Hadrian's Wall there were codes of law, habeas corpus, aqueducts and the literature of Cicero, Virgil and Tacitus -- and on the opposite side a violent, less sophisticated tribalism.

Hadrian assumed that there was a paradox about walls innate to the human condition. Scottish tribes hated Roman colonial interlopers and wanted them off the island of Britain. But for some reason the Scots did not welcome the wall that also stopped the Romans from entering Scotland.

The exasperated Romans had built the barrier to stop the Scots from entering Roman Britain, whether to raid, trade, emigrate or fight.

Today, the European Union has few problems with members that do not enforce their interior borders. But European nations are desperate to keep the continent from being overwhelmed by migrants from North Africa and the Middle East. Like the Romans, some individual EU nations are building fences and walls to keep out thousands of non-European migrants, both for economic and national security reasons.

Many Middle Easterners want to relocate to Europe for its material and civilizational advantages over their homes in Algeria, Iraq, Libya, Morocco or Syria. Yet many new arrivals are highly critical of Western popular culture, permissiveness and religion -- to the extent of not wanting to assimilate into the very culture into which they rushed.

Apparently, like their ancient counterparts, modern migrants on the poorer or less stable side of a border are ambiguous about what they want. They seek out the security and bounty of mostly Western systems -- whether European or American -- but not necessarily to surrender their own cultural identities and values.

In the case of Hadrian, by A.D. 122 he apparently felt that Rome's resources were taxed and finite. The empire could neither expand nor allow tribes to enter Roman territory. So his solution was to wall off Britain from Scotland and thereby keep out tribes that sometimes wanted in but did not wish to become full-fledged Romans.

The same paradoxes characterize recent, sometimes-violent demonstrations at Trump rallies, the controversy over the potential construction of a fence on the Mexican border 25 times longer than Hadrian's Wall, and the general furor over immigration policies.

Mexico is often critical of the United States and yet encourages millions of its own people to emigrate to a supposedly unattractive America. Some protestors in turn wave the flag of the country that they do not wish to return to more often than the flag of the country they are terrified of being deported from. Signs at rallies trash the United States but praise Mexico -- in much the same manner that Scots did not like Roman Britain but were even less pleased with the idea of a fortified border walling them off from the Romans.

What are the answers to these human contradictions?

Rome worked when foreigners crossed through its borders to become Romans. It failed when newcomers fled into the empire and adhered to their own cultures, which were at odds with the Roman ones they had ostensibly chosen.

There were no walls between provinces of the Roman Empire -- just as there are no walls between the individual states of America -- because common language, values and laws made them all similar. But fortifications gradually arose all over the outer ring of the Roman world, once Rome could no longer afford to homogenize societies antithetical to their own.

If Mexico and other Latin American countries were to adopt many of the protocols of the United States, their standard of living would be as indistinguishable from America's as modern Scotland is from today's Britain.

Or if immigrants from Latin America were to integrate and assimilate as rapidly as possible, there would be less of a need to contemplate walls.

Historically, as Hadrian knew, walls are needed only when neighboring societies are opposites -- and when large numbers of migrants cross borders without necessarily wishing to become part of what they are fleeing to.

These are harsh and ancient lessons about human nature, but they are largely true and timeless.