War is all about taking sides – unless, of course, you can't because you belong on both sides. That's the sort of conflict that citizenship laws were intended to avoid. But that's how Haider Thamir, a citizen of both Iraq and the United States, felt during Operation Iraqi Freedom. "It was like watching your mother and your father having an ugly argument," the Maryland-based insurance broker says quietly, "and there's nothing you can do about it."

Whatever else plays into nationality – ethnicity, your birthplace, the place you settle as an adult – what is most revealing is whom you identify with in times of conflict. In this mobile modern world, that core element of national identity is being challenged, as Haider Thamir knows.

Born in Baghdad in 1960 to an American mother and an Iraqi father, Thamir understood from childhood the challenges of negotiating a common path between two incongruent ways of life. This spring, 13 years after moving to the United States, when Thamir watched his two countries clash in war, he felt, he says, the mixed feelings that any immigrant would recognize.

But the dual citizen's divided sense of self differs from the transitional mix of nostalgia and anticipation that accompanied the immigrant's one-way passage from the old world to the new. That's because it reflects not only the past, but also the present and future. Dual citizens, according to the State Department's definition, "owe allegiance to both the United States and the foreign country." Thus, even though Thamir harbors no sympathies for Saddam Hussein, whose regime imprisoned and tortured his father, and even though his disagreement with America's intervention is based on widely held humanitarian concerns, his dual status is controversial. For some, such duality smacks of duplicity; for others, it holds the promise of a more unified world, in which narrow nationalistic concerns are yielding to the concept of universal human rights.

Dual citizenship upsets the traditional link between citizenship and allegiance to one nation, and it is particularly problematic, argues Mark Krikorian, executive director of the Center for Immigration Studies, in a country like the United States where commitment to an ideological proposition is a major component of national identity. That, says Peter Spiro, a professor of law at Hofstra University, is an old-world way of thinking, which fails to reflect the comings and goings and complex cultural identities of our modern, transnational world.

It is unrealistic to suppose that immigrants ever simply switched allegiance the moment they moved from one country to another, but as Thamir acknowledges, "today, the magnitude is different," as is the opportunity to retain formal ties of citizenship to more than one nation. About 100 countries, including the United States, now permit some form of multiple citizenship. Whether by birth, marriage or naturalization, millions of Americans are entitled to carry a second passport and even to fight another country's wars.

Not all of them are immigrants, of course. "The issue of nationality," says Benjamin Leavenworth, a 33-year-old cross-cultural business consultant, "is going to disappear." Leavenworth, whose family traces its American ancestry back through Civil War heroes to early colonists, describes himself as a "new kind of American." Born in Mexico City to an American father and a Swiss-American mother, Leavenworth carries passports from Mexico (which opposed the American intervention in Iraq) and Switzerland (which is traditionally neutral) alongside his American passport. "I can't help but look at politics from three different angles," he says.

War doesn't welcome such open-mindedness. Indeed, the threat of international conflict has been a powerful incentive for establishing a system of citizenship that fostered allegiance to a national cause and guaranteed individuals the rights and responsibilities of membership in a single state. As David Martin, a professor of law at the University of Virginia, points out, "However much one could tolerate complex and layered loyalties in times of peace, war may demand an unquestioning obedience." That is why a Hague Convention, written during the interwar years in 1930, opens with the phrase, "Every person should have a nationality and should have one nationality only." And it explains why acceptance of more than one citizenship spread during the optimism that followed the fall of the Berlin Wall.

The War on Terrorism and Operation Iraqi Freedom has renewed concerns about such liberal practices. Recently released draft legislation entitled the Domestic Security Enhancement Act – a follow-up to the 2001 Patriot Act – allows for an American to be expatriated, or stripped of U.S. citizenship, "if, with intent to relinquish his nationality, he becomes a member or provides support for a group that the United States has designated as a 'terrorist organization.'" (In April, Britain passed similar legislation and immediately used it to target the dual British-Egyptian citizen Abu Hamza, cleric of London's radical Finsbury Park Mosque.) In recent months, Iraqi Americans have been singled out for interviews with intelligence officials – a tactic that has evoked comparisons with World War II when Japanese and German dual citizens met with mistrust (and, in that era, mistreatment).

The dangers that crystallized during World War II of assuming any simple equivalence between ethnicity, citizenship and allegiance are if anything more pronounced today. Just take a look at the community where allegiance to a country's goals is most crucial – America's fighting forces. Almost 40,000 members, or three percent, of the active-duty military – men and women who are put their lives on the line for this county – are not even Americans. They are immigrants, wielding U.S. weaponry on their way to U.S. citizenship. The majority are Hispanics, following in the tracks, says Krikorian, of Irish immigrants in the Civil War, southern and eastern Europeans in World War I and of Japanese Americans during World War II.

The change these days is that when these men and women become Americans, many will retain their old citizenships. They'll become dual-citizen-soldiers, whose allegiance is – at least theoretically – divided. And they will join a body of soldiers who already held two or more citizenships when they enlisted.

The army doesn't keep track of how many dual citizens there are in its ranks, according to S. Douglas Smith, Public Affairs Officer at the U.S. Army Recruiting Command. All enlistees take an oath of allegiance to the Constitution, and dual citizenship only becomes an issue, he says, if they apply for jobs that require special security clearance. Although international law prevents a country from compelling soldiers to take up arms against a nation of which they are citizens, says Stephen H. Legomsky, Charles F. Nagel Professor of International and Comparative Law at Washington University School of Law, "they do so voluntarily all the time." Like its civilian population, America's fighting forces today are not just multicultural but multinational.

One small group illustrates the point. Back in April, when the five POWs from the U.S. Army's 507th Maintenance Division were welcomed home to Texas as American war heroes, news of their release reached their compatriots – in Panama, the Philippines, Mexico and New Zealand, as well as in the United States. One of the five was born in the Panama Canal Zone; another is of Filipino descent; a third was born in Texas to Mexican parents and is therefore eligible for Mexican as well as U.S. citizenship, although it was unclear during his captivity whether he actually had both; and a fourth, Sgt. James Riley, is a dual citizen of the United States and of a country whose leader voiced outspoken opposition to the American-led intervention in Iraq – New Zealand.

For most of U.S. history, dual citizenship spelled trouble, not so much because of fears that immigrants might lack commitment to their new homeland but because of the need to make Americans of them. Creating a nation out of immigrants involved releasing them from the ties that bound them to their countries of birth. That process challenged notions of perpetual allegiance that reigned in many countries, including Britain.

In the early-19th century, Britain refused to accept the right of its subjects to become Americans. Once British, always British, the thinking went, and British naval vessels acted on that assumption, stopping U.S. ships and seizing crew members believed to have been British. Claiming that these naturalized Americans still owed allegiance to the King, the British pressed them into service – a policy that contributed to the outbreak of war in 1812. Although Britain eventually gave up the notion of perpetual allegiance to the monarch, for years afterward, according to Spiro, naturalized Americans risked being conscripted if they returned to such countries as Prussia and Italy. Dual citizens could also present difficulties, as President Ulysses S. Grant recognized, if they used "the claims of citizenship of the United States simply as a shield from the performance of the obligations of a citizen elsewhere."

Hence the State Department's insistence, through the middle of the last century, that people who were born into two citizenships choose one when they reached adulthood and that women who married into a second citizenship relinquish their old allegiance. That approach failed to hold up in the courts, and the United States has resorted to a "don't ask, don't tell" policy – with few, if any, consequences for "telling." Other countries, including Mexico, recognizing the economic benefits of maintaining formal contact with their populations overseas, are actively encouraging dual citizenship.

All of which upsets the old world order in which citizenship acted as a sort of "international filing system, a mechanism for allocating persons to states," in the words of University of California sociologist Rogers Brubaker. It disrupts the traditional link of privileges and protections that bound a sovereign nation and its citizens, complicating the State Department's avowed mandate "to protect American interests and citizens around the world and at home." Dual nationality, the department warns, may "limit U.S. Government efforts to assist citizens abroad."

And it's easy to see why. Dual citizenship raises the specter of redundancy and confusion, of individuals gaining assistance from two governments or from none. Conscription is no longer the primary concern now that few countries have a draft. But dual citizens can get into other kinds of trouble – over child custody after international divorces, for example, or taxation.

There are also occasional skirmishes about the use of a second passport as a kind of get-out-of-jail-free card. In 1997, 17-year-old Marylander Samuel Sheinbein committed a brutal murder then fled to Israel, which refused to extradite him because he is the son of a man who claims Israeli (as well as American) citizenship. Although Sheinbein was tried in Israel, his 24-year prison term is probably more lenient than the sentence he would have received in this county.

Governments, meanwhile, respond idiosyncratically to the needs of their people overseas, even in highly publicized cases, such as that of the POWs, according to several news accounts. The Foreign Secretary of the Philippines reportedly appealed to the Iraqi administration on behalf of Spc. Joseph Hudson out of a sense of ethnic solidarity, even though he is not a Philippine citizen. The Mexican government, which has been extending its outreach to the 20 million Mexicans living overseas, reacted similarly to Spc. Edgar Hernandez's plight. But the New Zealand government showed little interest in intervening with the Iraqi regime on behalf of its citizen, Sgt. Riley, when he was captive in Iraq. "There would be concern about any citizen being in harm's way, but when free individuals go, presumably get the sort of immigration status they require," said New Zealand defense secretary Mark Burton, once they have "…enlisted in the armed services, presumably sworn loyalty to that country…they are engaged in the business of that state."

An understandable reaction, perhaps, given that Riley has not lived in New Zealand since he was 10. But, to some, it seemed like an act of betrayal of the bond between state and citizen, as one British visitor to New Zealand revealed in an impassioned letter to the Christchurch Press newspaper: "The way this country's government has distanced itself from one of its sons (James Riley) really sickens me.

I would rather go back to England. It may be overpopulated and polluted but at least it doesn't turn its back on one of its own."

The weakening bond between sovereign state and citizen has been accompanied by the rise of the human rights regime, which diminishes some such concerns about diplomatic protection because it supposes that all human beings deserve basic rights regardless of their citizenship. During the POWs' captivity, President Bush himself turned to universal rights, angrily demanding that the American soldiers be treated in accordance with the Geneva Conventions, which set humanitarian rules for the conduct of war that apply to anyone.

Universal rights, which elevate the notion of "personhood" over "citizenship," mean that states can no longer treat their own citizens as they wish. "Germany can't claim a right to mistreat a German," says Spiro, "and to the extent it does, the rest of the world has a right to complain about it."

The change is an important one. Citizenship, like membership in any other group, means something different depending on whether you are inside or outside that group. While it helps to foster a sense of solidarity and community, citizenship can also act as an exclusive club. "In its worst forms, this use of citizenship helps deny the very humanity of the non-citizen," writes Martin, recalling a tough lesson the United States learned in the late 19th century. Even though the country was founded on the Jeffersonian ideal of "unalienable rights" for all, the 1857 Dred Scott decision denied blacks citizenship, preventing them from becoming "constituent members of this sovereignty" and propelling the country toward civil war. The Fourteenth Amendment repealed that ruling, placing the emphasis back on the equal rights of all human beings and providing a template for the 1948 Universal Declaration of Human Rights and its assertion of "the equal and inalienable rights of all members of the human family."

Not all countries abide by the notion of human rights, though. Haider Thamir knows that all too well. "I know people who disappeared in Iraq," he recalls. "I lived with anxiety about that. It's terrible."

Although Thamir feels torn between his two countries of citizenship, his feelings about the values on which those countries are based are unambiguous: "I know a lot of people – whether they are Lebanese or what have you – who made up their mind to have a citizenship in a country that respects human rights. It's essential."

Values such as these, which transcend national borders and ethnic enclaves, bridge the cultural divides that are the inevitable experiences of many citizens of our modern, mobile world. Now that we no longer fit into the international filing system that citizenship once was, universal human rights offer the prospect of a new world citizenship, one in which people like Haider Thamir and Benjamin Leavenworth won't be expected to choose which side they're on according to the passports that they carry.