When the Mexican government realized it could benefit from this as well,  it changed its own citizenship law to allow Mexicans to be dual citizens of both the U.S. and Mexico, for the benefit of Mexico. 
In 2003, when the U.S. invaded Iraq, the Mexican government tried to exploit the situation. It announced a census of Mexican citizens  (and soldiers of Mexican ancestry) in the U.S. military and negotiation with Saddam Hussein for the release of American prisoners of war who were also Mexican citizens!
Nothing much came of it, but it illustrates the vast possibilities  for meddling inherent in widespread dual citizenship.
Does U.S. law allow dual citizenship? You wouldn't get that impression from the oath of citizenship.
When an immigrant becomes a citizen of the United States, he swears an oath of allegiance. Here is what the new citizen promises: 
"I hereby declare, on oath, that I absolutely and entirely renounce and abjure all allegiance and fidelity to any foreign prince, potentate, state, or sovereignty of whom or which I have heretofore been a subject or citizen; that I will support and defend the Constitution and laws of the United States of America against all enemies, foreign and domestic; that I will bear true faith and allegiance to the same; that I will bear arms on behalf of the United States when required by the law; that I will perform noncombatant service in the Armed Forces of the United States when required by the law; that I will perform work of national importance under civilian direction when required by the law; and that I take this obligation freely without any mental reservation or purpose of evasion; so help me God." [ Oath of Allegiance--USCIS Website ]
But in fact, because of our legislating Supreme Court, dual citizenship is now law of the land.
As the website of the U.S. embassy in Mexico  announces to the world:
"Being a dual national and carrying the passport of another country is perfectly legal. Under U.S. law, naturalizing as a citizen of a foreign state will not in and of itself cause you to lose your U.S. citizenship." [Dual Nationality ]
The Selective Service System even exempts some dual citizens  from registration.
In the old days, when citizenship meant something, the United States had rules to control this sort of thing. There were "expatriating acts" which could cause you to lose your citizenship.
Thus Clemente Perez, a U.S.-born citizen of Mexican ancestry, moved to Mexico before World War II and voted in a Mexican election. In Perez vs. Brownell  (1958) the Supreme Court ruled 5-4 that an American citizen who voted in a foreign election could lose his citizenship.
But a scant nine years later, in the landmark Afroyim vs. Rusk  case (1967), the court ruled 5-4 that Beys Afroyim, a naturalized citizen, did not renounce his American citizenship by voting in an Israeli election . This invalidated Perez vs. Brownell.
U.S. law followed suit (the denaturalization laws were repealed  in 1978, since they were a dead letter as soon as the Supreme Court ruled), and now it's almost impossible for an American citizen to lose his citizenship, no matter what he does in a foreign country.
"Military service in foreign countries usually does not cause loss of citizenship since an intention to relinquish citizenship normally is lacking." [Possible Loss of U.S. Citizenship and Foreign Military Service ]
"Currently, there is no general prohibition on U.S. citizens' running for an elected office in a foreign government." [Advice About Possible Loss Of U.S. Citizenship And Seeking Public Office In A Foreign State .]
So how could a dual citizen of the U.S. and another country lose his American citizenship?
The State Department has a document entitled "Possible Loss of U.S. Citizenship and Dual Nationality ." It does list possible expatriating acts—being naturalized in a foreign country, serving as an officer or NCO in a foreign army, being employed by a foreign government, and even serving in a military engaged in hostile action with the U.S. or being convicted of treason. Yet it adds:
"the actions listed above can cause loss of U.S. citizenship only if performed voluntarily and with the intention of relinquishing U.S. citizenship. The Department has a uniform administrative standard of evidence based on the premise that U.S. citizens intend to retain United States citizenship when they obtain naturalization in a foreign state, subscribe to routine declarations of allegiance to a foreign state, or accept non-policy level employment with a foreign government."
In other words, you can commit a possibly expatriating act and still not lose your citizenship. In fact, the State Department assumes people will do these things and still be U.S. citizens. The key concept is "intent." Even if you say to a foreign government that you're renouncing your U.S. citizenship it doesn't count on the U.S. side unless intent to renounce can be proved.
Nowadays, the only way to lose American citizenship is by
"making a formal renunciation of nationality before a diplomatic or consular officer of the United States in a foreign state , in such form as may be prescribed by the Secretary of State." [Renunciation of U.S. Citizenship ]
That's it. That's the only way to renounce citizenship.
And practically speaking, it hardly happens anymore. (A rare example is the recent Yaser Hamdi  case, involving a dual U.S./Saudi national captured in Afghanistan  while serving in the Taliban. Hamdi was a U.S. citizen by virtue of his having been born in the U.S ., and as part of his repatriation to Saudi Arabia was required to renounce his U.S. citizenship. )
Today there are millions of dual citizens of the U.S. and other countries. And not only Mexico .
Foreign governments  and immigrants are learning more and more ways to leverage dual citizenship and game the system.
Is it wise to open the immigration door  even wider, when we haven't really straightened out the citizenship issue?
Don't all Americans have a stake in the citizenship question—including those of us who are (unfashionable as it may now be) merely U.S. citizens?
American citizen Allan Wall (email  him) resides in Mexico, with a legal permit issued him by the Mexican government. Allan recently returned from a tour of duty in Iraq with the Texas Army National Guard. His VDARE.COM articles are archived here ; his FRONTPAGEMAG.COM articles are archived here  his "Dispatches from Iraq" are archived here  his website is here .