Even before President Trump invoked powers under the 1798 Alien Enemies Act, the historical meaning of “alien enemy” had become a topic of controversy, because the distinction between an alien enemy and an alien friend is also relevant in the context of immigration and birthright citizenship.

　　Largely overlooked in this debate has been the legal doctrine that defines the difference between an alien enemy and an alien friend: the doctrine of allegiance, which our Founders inherited from English law. The English doctrine closely paralleled the international law concept of the same name.

　　To understand the Alien Enemies Act, it is important  to understand the Founding-era rules of allegiance and how they divide alien friends from alien enemies. We cannot here fully address the president’s deportation power, because the Alien Enemies Act does not authorize presidential deportation of all alien enemies—only those participating in an “invasion or predatory incursion … by a foreign nation or government.” It is nevertheless a useful starting point to recover the historical meaning of this distinction.

　　The doctrine of allegiance is crucial for understanding the Alien Enemies Act, but it also is crucial for understanding the Constitution itself. The meaning of many of that document’s terms rests in part on the meaning of allegiance. Examples include the Define and Punish Clause (Article I, Section 8, Clause 10), which grants power to Congress to regulate immigration and emigration; the requirement that the president be a “natural born Citizen” (Article II, Section 1, Clause 5); the provisions addressing treason (Article I, Section 6; Article II, Section 4; Article III, Section 3 Article IV, Section 2, Clause 2); and federal and reserved state war powers (Article I, Section 8, Clause 11, Article I, Section 10, Clause 3, and others). And as the Supreme Court recognized in United States v. Wong Kim Ark (1898), allegiance also is central to the meaning of the 14th Amendment’s phrase “subject to the jurisdiction thereof”—and therefore to the meaning of birthright citizenship.

　　Despite its importance, few modern constitutional commentators have grasped the actual meaning of “allegiance” as the Founders understood it. In part, I think, this is because few commentators have much of a background in eighteenth-century law. In part also, it is because crucial English precedents on the subject—for example, much of Calvin’s Case (1608)—are composed partly in untranslated Latin, and, dedecore dictu, most commentators are ignorant of that language. Whatever the reasons for modern lapses, to fully understand the Constitution—or the Alien Enemies Act—you have to know the Founding-era rules of allegiance. My coauthor Andrew T. Hyman and I confirmed this while researching the scope of the war powers the Constitution reserves to the states for an article published last year in the British Journal of American Legal Studies, in which we reconstructed the Founding-era meaning of allegiance. Readers interested in more information—including some of the errors made by modern writers—may wish to consult that article.

　　In Anglo-American law, allegiance was a relationship between a person and a sovereign: The person owned loyalty and obedience to the sovereign and the sovereign owed protection to the person. (The feudal origins of the doctrine are obvious.) In Anglo-American law, a person in allegiance to a sovereign was denominated a subject of that sovereign.

　　After Independence, Americans became subjects of states in which they resided. But the monarchical term “subject” was distasteful to Americans. Hence, Article IX of the Articles of Confederation spoke in a more republican fashion of “members of a state.” But the concept was the same. For the sake of simplicity, I’ll continue to use the word “subject” rather than “member.”

　　Note that the term “subject” was not the same as the republican term “citizen.” One could be in allegiance to a republic—and therefore a subject—without being a citizen.

　　Whether or not one was a subject of a sovereign, and therefore in allegiance, depended on several factors. They included place of birth, current location, intent or consent to give allegiance, status of the father, status of the mother, and a person’s conduct. No one factor necessarily controlled.

　　English law recognized four categories of subjects: natural born, naturalized, denizens (aliens with special papers), and resident alien friends. We shall leave the first three categories for later examination, and focus on aliens.

　　The word “alien” was a synonym for foreigner. Resident alien friends were foreigners who were not classified as alien enemies and entered and remained in the host country under circumstances implying submission to its laws. When a resident alien friend entered a host country, he or she entered into local allegiance to the host. He or she thereby became a subject of the host country, and generally remained one for the duration of the stay.

　　Foreign diplomats represented an exception to this rule: A foreign diplomat was said to carry around a piece of his nation with him. He was a friendly alien, but not in allegiance to the host country.

　　Those debating whether the Act can be invoked should focus on whether those the president has ordered deported are participants in an invasion or predatory incursion sponsored by a foreign government.

　　The resident alien friend continued to owe allegiance to his natural sovereign, and thus continued as a subject of that sovereign. Moreover, allegiance to one’s nation of origin superseded local allegiance. But this was not a problem as long as the nation of origin and the host country were on good terms so that the two sets of duties were not inconsistent.

　　All subjects, including alien friends, who seriously breached their duty of allegiance were potentially guilty of treason. A non-subject could not be tried for treason, no matter how heinous his conduct. Nonetheless, status as a subject brought certain benefits. The most important was the claim on the host country for protection. In England, this included the privilege of the writ of habeas corpus—which was reserved for subjects.

　　The most famous English habeas corpus case of all was initiated by a resident alien friend. He was James Somerset, an African who had been transported to Virginia to serve as a slave. Somerset’s putative owner then took him to England but kept him imprisoned aboard ship. Somerset escaped and submitted himself to English jurisdiction and to allegiance to the Crown. This entitled him to maintain a habeas corpus action. In Somerset v. Stewart (1772), the petitioner’s counsel famously argued, “The air of England is too pure for a slave, and every man is free who breathes it.” The Court of King’s Bench agreed.

　　As Somerset v. Stewart illustrates, under Founding-era law, a sojourner as well as a permanent resident could qualify as a resident alien friend. In other words, if otherwise qualified, even a foreign tourist who stays in the United States for a week or two is an alien friend and in local allegiance to this country.

　　One could not, however, be a resident alien friend if one was an alien enemy. What was an alien enemy?

　　As a general rule, a foreigner from Nation A was an alien enemy to Nation B if the two nations were at war. This was true whether or not the foreigner was physically present in Nation B. One should not deduce, however, that a resident was an alien enemy only if he was from a nation with which the host country was at war. This would overlook some wrinkles in the Founding-era law of allegiance:

　　First: Under some conditions, a person from a hostile foreign country could enjoy treatment as a resident friendly alien. This might happen if war broke out between a resident alien’s nation of origin and the host country. The necessary conditions were that (1) the alien petitioned to stay in the host country, (2) the host country consented, and (3) the alien comported himself well and did nothing inconsistent with local allegiance. If he broke his obligation of allegiance, the host country could treat him either as a traitor or as an enemy alien.

　　During the Founding era, the required permission to remain customarily was granted only for a few weeks, so the departing foreigner could wrap up his affairs. Again, friendly alien status did not depend on the duration of one’s stay.

　　Second: A person from a friendly foreign nation who participated in a hostile nation’s attack on his host country was an alien enemy of the host country. The fact that he was from a friendly nation didn’t matter. Vaughan’s Case (1696) so held and in Miller v. United States (1870), the Supreme Court endorsed Vaughan’s Case.

　　Third: Bad conduct could convert a resident alien friend into an alien enemy, irrespective of his nation of origin. As Emer de Vattel, the Founders’ favorite authority on international law, put it: “Whoever offends the state, injures its rights, disturbs its tranquility, or does it a prejudice in any manner whatsoever, declares himself its enemy, and puts himself in a situation to be justly punished for it.”

　　Fourth: As Vattel and other international law scholars emphasized, control of national borders was a recognized prerogative of sovereignty. A person who entered a host country illegally—even from a friendly nation—could not qualify as an alien friend, and was, therefore, an alien enemy.

　　The term “alien enemies” in the Alien Enemies Act includes 1) foreigners in the United States from nations with which we are at war where no specific permission has been granted to stay; 2) foreigners who participate in an attack on the United States, irrespective of whether the United States is at war with their nation of origin; 3) foreigners who otherwise conduct themselves in ways inconsistent with allegiance to the United States; and 4) foreigners who entered the United States illegally.

　　As noted above, though, status as an alien enemy is not sufficient to trigger the Alien Enemies Act. Those debating whether the Act can be invoked should focus on whether those the president has ordered deported are participants in an invasion or predatory incursion sponsored by a foreign government.