The hottest flash point at the moment in this summer’s immigration debate is birthright citizenship. Should children born in this country to non-citizen parents be nevertheless entitled to citizenship? As President, Donald Trump has promised to end this feature of the immigration laws as a hallmark of his anti-immigration policy. Most of the Republican primary candidates have fallen in line behind him. But as was quickly pointed out, this seemingly defies the 14th Amendment, which begins “All persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States and of the State wherein they reside.” The 14th Amendment constitutionalized what is known as “jus soli”, a feature of English common law going back centuries, saying that anyone born within the country is a citizen of the country.

Immigration critics contend, however, there is a loophole. The words “subject to the jurisdiction” create a limit on birthright citizenship. If someone is not “subject to the jurisdiction” of the United States, then they are not entitled to citizenship at birth. So, the reasoning goes, if illegal immigrants are deemed not “subject to the jurisdiction” of the United States, then their children need not be given citizenship at birth.

To make this argument work, legal scholars have argued that “subject to the jurisdiction” was meant to echo a parallel exception in the 1866 Civil Rights Act. The Civil Rights Act guaranteed citizenship to anyone born in the country, whose parents were not “subject to any foreign power” or to “Indians not taxed.” Hence, the exception for those “subject to the jurisdiction” was intended to carry over the exception for independent Indian tribes and those “subject to any foreign power.” Because non-resident aliens have foreign citizenship and are consequently subject to a foreign power in the colloquial sense, jus soli should not apply.

Scholars of the 14th Amendment have bandied about its legislative and drafting history to support any number of readings of drafting intent for the Citizenship Clause. And at the end of the 19th century, the Supreme Court decided a few cases that seemingly cut both ways. On the one hand, the Court concluded that children born into independent Indian tribes were not entitled to citizenship because, to be subject to the jurisdiction of the United States required that their parents owe “direct and immediate allegiance” to the United States. Under the peculiarities of Indian law, they did not. And in the Slaughterhouse Cases, the Court opined in dicta that “‘subject to its jurisdiction’ was intended to exclude from its operation children of ministers, consuls, and citizens or subjects of foreign States born within the United States.” But, on the other hand, when the question was actually presented in the case of a child of Chinese immigrants, the Court concluded that jus soli applied and that even the children of non-citizens enjoyed birthright citizenship.

So what does “subject to the jurisdiction” really mean? Those seeking to resurrect the arguments for reconsidering birthright citizenship focus the fact that non-resident aliens owe “allegiance” to a country other than the United States. This intuitive point, they suggest, favors a broad reading of the Citizenship Clause’s exception. The best summary of the constitutional foundation of jus soli is an OLC opinion by Walter Dellinger from the 1990s, the last time birthright citizenship came under serious scrutiny. This will sound archaic – because it is – but Dellinger concludes the phrase excludes only the children of foreign diplomats, sovereign Indian tribes, and occupying enemy armies. (The Fourteenth Amendment also has been interpreted to exclude children born in American colonies, like Puerto Rico and American Somoa, but that is because the colonies are deemed to be technically not part of the United States.)

What a strange exception? Why would such a picayune exclusion make its way into the otherwise sweeping language of the 14th Amendment? This idiosyncrasy certainly lends some force to the arguments of those who contend that “subject to the jurisdiction” should be interpreted more broadly. The problem is that, dicta from the Slaughterhouse Cases aside, the narrow reading is rooted in the importance of what being a “subject” means when dealing with questions of nationality and “allegiance,” particularly in wartime.

In the middle of the 19th Century, being “subject to the jurisdiction” of the United States versus being “subject to a foreign power” had very specific national security consequences. Indeed, the language had nothing to do with immigration. As even critics of modern immigration policy admit, the 14th Amendment was passed at a time when there were basically no immigration laws. It was a time of open borders. Citizenship, to be sure, held significance in terms of political rights and duties. But, as a general rule, individuals could come and go at will. Being “subject” to the laws of a particular power was a question of “jurisdiction” in its oldest and most technical sense. To what government does one owe allegiance in exchange for legal protection? And as a matter of U.S. constitutional history, mere presence, regardless of citizenship, was sufficient. It did not matter that you might owe allegiance by virtue of your citizenship to a foreign country. While you were here, you enjoyed the full protection of American laws and owed your full allegiance in return, such that you could even be prosecuted as a traitor in times of war.

If that sounds oxymoronic, that the foreign citizens could be tried for treason, there is a long history of it. There were at least two cases during the Civil War that reached the Supreme Court and held precisely that. In one case British citizens were supplying saltpeter to the Confederacy in Alabama. In another, a Russian was smuggling for the Confederacy in Texas. In both, the Supreme Court deemed it uncontroversial that “All strangers are under the protection of the sovereign while they are within his territories, and owe a temporary allegiance in return for that protection.” As soon as you entered the country, in other words, you were a “subject” owing full allegiance to the United States and therefore under its “jurisdiction.”

This rule even applied to enemy aliens as recently as the 1940s. In cases that will be familiar to Lawfare readers who follow Guantanamo, German saboteurs snuck into the country and were tried by military commissions for, among other offenses, aiding the enemy. Aiding the enemy is the military version of treason. Precisely because these Germans had entered within the borders of the United States without disclosing that they were agents of a foreign power, they were subject to our laws and their efforts to aid the Nazi cause were treasonous. If that sounds counterintuitive, you only have to imagine a situation in which one of the saboteurs, while walking around Manhattan before being exposed, got mugged. It would have been the New York City police to answer the call. And the quid pro quo for that protection was the demand for allegiance that made him a subject of the jurisdiction so long as he remained within it.

This rule goes back far into English common law. A century before the Civil War, the leading English treatise on treason, Sir Michael Foster’s “Discourse on High Treason,” explained that if an alien “committeth an Offence, which in the Case of a Natural-born Subject would amount to Treason, He may be dealt with as a Traitor. For his Person and Personal Estate are as much under the Protection of the Law as the Natural-born Subject’s: and if He is injured in either, He hath the same Remedy at Law for such Injury.” The key was that presence within the realm made one just as subject to the laws, its benefits and burdens, as someone who owed that allegiance from birth. And it should not be surprising that the kind of allegiance necessary to support a charge of treason would bear a close relationship to the qualifications for citizenship. Committing treason is one of the few ways citizenship can be lost.

The major exception to this territorial duty of allegiance is for diplomatic personnel. They, like invading armies, never claim the protection of the local laws and never incur any allegiance to it. When serving a nation at war with the crown, these individuals were deemed by Foster to be “Enemies subject to the Law of Nations; never as Traitors subject to our Municipal Laws.” Diplomatic personnel are not even subject to our day-to-day municipal laws, as the New York City parking authorities responsible for policing the area around the United Nations will attest. Hence, when the 1866 Civil Rights Act used the phrase “subject to any foreign power,” it did not mean being a mere citizen of a foreign power. If that was what the drafters intended, they would have just said “citizen,” which was a term of art at the time implicating a permanent duty of allegiance. Instead, the exclusion was predicated on the fact that genuine subjects of a foreign power only had a duty of allegiance to their home country and, reciprocally, could only claim legal protection as a matter of our country’s willingness to afford diplomatic comity.

The 14th Amendment therefore reflects a critical trade-off in the law of “allegiance” and “jurisdiction” that was more salient when it was written than it may be today. And contrary to modern immigration critics, mere presence was sufficient to require allegiance and confer the reciprocal legal benefits you got in return. An individual who comes into this country enjoys the protections of our law regardless of how they got here, whether they entered illegally, overstayed a visa, or smuggled themselves in under orders from Hitler to destroy our steel mills. By the same token, they owe allegiance to our country while they are here and are liable to suffer the burdens of that allegiance, whether it is paying sales tax or being tried as a traitor.

That trade-off may seem to some too generous today. But as a historical matter, national security benefited from demanding allegiance from as many people as possible and, in particular, from anyone within your geographical borders. That is one of the reasons why the 14th Amendment’s exception is so narrow. And it is also why ending birthright citizenship will require a constitutional amendment or a judicial abandonment of what “subject to the jurisdiction” meant when it was written.