In the days after the filmmaker Theo van Gogh was murdered in Amsterdam by a Dutch citizen who carried Moroccan citizenship as well, Jan-Peter Balkenende, prime minister of the Netherlands, promised a "hard-line approach". It would include legislation to strip Dutch nationality from dual citizens who commit terrorist crimes. The policy reversal was sudden, as for decades the Dutch state had smiled on dual nationality. But such reversals are now the norm.

The five years since Germany passed its first laws permitting dual citizenship have been marked by frequent appeals from politicians to refocus on German national pride. Increasingly the US is insisting that its own dual nationals travel on their American passports.

Dual citizenship is not about to disappear. Many who possess it are the product of international marriages, and could be deprived of it only through bureaucratic high-handedness. But mass dual citizenship, of the sort that has been encouraged as a way of building bridges between cultures and honouring ancestries, is destined to wither, and will be seen in retrospect as a fad.

Foes of dual citizenship often stress instances of dual loyalty, such as the case of Tomoya Kawakita, a Japanese-American dual citizen who spent the second world war in Japan torturing American soldiers in a prisoner-of-war camp. On his return to the US after the Japanese defeat, he was spotted by one of his victims and tried for treason. His defence was that a dual citizen owes his loyalty to the country he finds himself in, and that he had been free, as the citizen of a Japan at war, to torture enemy prisoners. US Supreme Court justice William Douglas said: "One who wants that freedom can get it by renouncing his American citizenship. He cannot turn it to a fair-weather citizenship, retaining it for possible contingent benefits." Mr Kawakita received the death sentence but was eventually sent back to Japan.

But such cases are rare. As Justice Douglas sensed, the overarching problem with multiple citizenship is not disloyalty but unfairness. Samuel Huntington, the Harvard political scientist, would agree. He takes a dim view of the proliferation of "ampersands" (his term for dual citizens), for whom US citizenship can serve as a mere benefit-generating supplement to some other affiliation.

Ampersands can enjoy more citizens' rights than their native-born American compatriots. Mr Huntington notes that a citizen who splits his time between Boston and Santo Domingo is permitted a democratic say in his second home. A citizen who splits his time between Boston and New York is not. Theoretically, children of two dual citizens can claim all four citizenships. They can be educated in the country that offers the best tuition aid, get healthcare in the country that subsidises it most generously and pay taxes in the country that collects the least. They can also clear out of any country facing hardship or an emergency, such as war.

Not just individuals but also governments in developing countries have come to see dual citizenship as a resource.

This may be why certain governments that recently rejected dual citizenship now promote it. In 1998, Mexico announced an amnesty to permit Mexican-Americans to reclaim citizenship (and invest money) there. Earlier this year, India introduced the category of "overseas citizen" and made it available to citizens of more than a dozen countries, not the ones where the most overseas Indians reside but the richest ones in which Indians reside. Strangely, this "citizenship" does not permit its bearers to vote, which does not sound like citizenship at all.

In practice, benefit-shopping has limits. But it exists, because under the conditions of globalisation, the rights of dual citizenship can travel more easily than the responsibilities.

Consider Samuel Sheinbein, a teenager from Washington, DC, who in 1997 murdered a 19-year-old neighbour, dismembered the corpse with a power saw, torched the remains and tried to hide the mess in a garage. By the time police figured out what had happened, Sheinbein was with his Israeli-born father in Israel, a country that does not extradite its citizens. Sheinbein was tried and received a sentence that was harsh by Israeli standards. But he is already eligible to apply for short-term release. His accomplice, who had no such option of selecting his legal venue, committed suicide in jail before he could be exposed to the considerably more ferocious justice of the American South.

This is exactly the "fair-weather citizenship" Justice Douglas feared. That it would eventually generate anger was inevitable. But worries over international terrorism have added urgency to the anger. Patrick Weil, a historian at the Sorbonne who studies comparative citizenship practices, notes that a few western states have mechanisms of denaturalisation for dual citizens, which apply in various limited circumstances.

Fear of terrorism has led many states, most recently the Netherlands, to re-examine their laws for possible use in determining who belongs and who does not. France can strip citizenship from those who become agents of another state, but this is not of much use in a war against non-state actors such as terrorists. The US can denaturalise citizens it has previously naturalised if their petitions were made fraudulently, and has been willing to use that power, notably in the case of former Nazis.

New US citizens must take an oath to "renounce and abjure absolutely and entirely all allegiance and fidelity to any foreign prince, potentate, state or sovereignty of whom or which the applicant was before a subject or citizen". It may be that the "sovereignty" in question can be taken to embrace terrorists.

In tranquil times, the occasional injustices of dual citizenship do not weigh so heavily against the consolations it offers, for instance as a means of acknowledging citizens' ancestral cultures. But in insecure times, that vast majority of citizens who have only one country are inclined to insist that citizenship be a matter not of culture but of allegiance. The writer is a senior editor at the Weekly Standard

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