Once again, the rich world is gripped by terror that the poor world will invade it. There is talk of "Fortress Europe", of redefining the 1987 Single European Act to deny freedom of movement to non-EU citizens. There are even fresh doubts about the wisdom of enlarging the Union eastwards. Will Poland be able and willing to run a tight external frontier and keep the indigent Asian hordes out of "civilised" Europe?
This is trash - but cyclical trash: a recurrent nightmare. We must have short memories if we do not remember the last grand panic. As the Soviet Union began to collapse, it was fashionable to predict that tens of millions of Russians would at any moment begin to pour westwards in search of bread and sausage. The barbarian invasions were about to repeat themselves. European culture and prosperity would be swamped, and a new Dark Age would begin.
But nothing of the sort happened. Russians do not leave their own country in bad times, but dig in and survive. There was a substantial tide of migrants into Poland, but they did not settle; they provided a source of cheap labour - mostly illegal - which, if anything, gave the Polish economy a boost. Later there was an influx of Romanians, many of whom reached Germany by way of Austria, but this has been absorbed. Wild suggestions that Nato might be converted into a frontier guard, lined up along the Oder and Danube rivers to collar the swimming tribes of Asia, faded away.
This is not to deny that there are some real problems. It was right that the Royal Navy should hunt down slave ships trying to smuggle "illegal immigrants" into the Americas after the abolition of the slave trade. And it is right that European nations should send warships and aircraft to intercept the mafia-run vessels carrying Kurds, Albanians or Bangladeshis across the Mediterranean. The only real difference is that the new slaves have been forced to pay extortionately for their passage. But the central problem is one of perception. We are refusing to look at the migration phenomenon as a whole.
Europeans, and Americans too, desperately try to categorise those who arrive from the poor world into legal or illegal immigrants, genuine asylum seekers (good) or "economic migrants" (bad), refugees entitled to shelter or "habitual overstayers" who intend to live off welfare. In the end these distinctions are absurd. Take, for example, the outburst last week by Klaus Kinkel, the German foreign minister. He complained that over 80 per cent of the 17,000 Turkish citizens who applied for political asylum in Germany last year were Kurds - and that most of them did not come from the war-torn south-eastern provinces but from big Turkish cities. Is Mr Kinkel seriously implying that he is dealing with 13,500 Kurdish liars, crooks and parasites?
Everyone who deals with immigrants and their problems knows that our categories are really nonsense. This is glaringly obvious in the arguments about asylum. There are times - the Cold War was one - when the difference between victims of political persecution and people who "merely" want a better life seems obvious. But even then the distinction was not that clear. It was not just that almost everyone who came from "the Communist bloc" was in some sense escaping a zone of political oppression. It was also that the mere act of applying for political asylum created the grounds for granting it. News of an application always got back through the Iron Curtain, and an unsuccessful applicant who returned home could expect to lose his or her job, to be denied a passport, to have parents thrown out of their flat or siblings barred from higher education.
When it comes to far bigger immigration flows from unhappy countries in this decade - Nigeria, Algeria, Turkey, Bangladesh, to name only a few - the distinction gets even more arbitrary. Of course we should shelter political fugitives. But why does that mean that "economic migrant" has to become a dirty term? Why is it respectable to rescue oneself from torture by electrodes, but disreputable to rescue one's family from torture by unemployment, debt-slavery and hunger which kills children slowly?
On the other side of the world the crash of the "tiger economies" is beginning to displace a torrent of poor immigrants whose cheap labour is no longer required. Thailand and Malaysia are preparing to expel some 2.5 million foreign workers and their families. South Korea wants to repatriate 270,000 foreigners. They will be returned to Indonesia, Burma, Bangladesh, India and other poor countries where they will be dumped back into penury. This enormous tragedy, far worse than anything that will happen to the citizens of the bankrupt "tigers", ought to move us to help. But it should also teach us, in Europe, the true nature of migration as a force in modern history.
Every focus of prosperity is like a furnace. It creates a huge updraft - and sucks the poor and uprooted towards it. At first this only happened inside nations, as the industrialisation of Britain or Germany sucked the rural population into new manufacturing cities. Now it happens on a supranational scale. The industrial "golden triangle" of Western Europe in the 1960s, centred on West Germany, pulled in the peasants of Portugal, southern Italy, Croatia, Greece and eventually Turkey. The boom around the Pacific Rim in the last decade attracted millions of the Asian poor. And the arrival of those migrants - "economic" indeed - was crucial for sustaining growth in those regions. Without their cheap labour, Frankfurt or Zurich, Hong Kong or Seoul would not glitter and roar. They, the "economic migrants", made our office towers and motorways, our cars and subway trains.
But capitalism has cycles. The furnace cools off. Then comes the question of what to do with the "economic migrants", those who are called "guest workers" when they are needed and "welfare parasites" when they are not. The demand for labour has fallen, and angry voices demand that they be sent back where they came from.
Exporting their own unemployment is the dream achievement of all free-market systems. But only apartheid South Africa really mastered it. As demand rose, contract labour was imported from puppet Bantustans. When it slackened, the blacks were deported to rot on their barren, eroded homelands. But only a police state can keep adjusting the "labour pump" in this way. In West Germany, when the boom years ended in 1973, the pump leaked and hundreds of thousands of migrant workers stayed on. In Asia today even the most authoritarian regimes will find it hard to expel all their surplus labour.
And the people of the poor world will keep on coming to the rich world. They will come faster when we need them again, but they will come anyway. The "global economy" also means that most poor peasants can find a labour contractor who will smuggle them to Europe. And no fortress walls, frontier controls or jails for asylum seekers will, in the end, keep them out.