A number of crucial issues in immigration law and policy now turn on the meaning of a previously little-noticed term in the Constitution: “invasion.” The Trump administration and some red state governments claim that illegal migration and drug smuggling across the southern border qualify as “invasion” under the Constitution and the Alien Enemies Act of 1798. If this ill-founded argument is not rejected by the courts, it would have dangerous and far-reaching implications. Among other things, it would empower the federal government to detain people without charge or trial, and open the door for states to engage in war with neighboring foreign nations without congressional authorization.
In a newly issued executive order, President Donald Trump claims that the illegal migration qualifies as “invasion,” thereby giving him the power to shut down all or nearly all legal pathways to migration across the border, even in violation of laws enacted by Congress. He cites the Guarantee Clause of Article IV, Section 4 of the Constitution, which states: “The United States shall guarantee to every State in this Union a Republican Form of Government, and shall protect each of them against Invasion.”
Trump also claims that the supposed “invasion” authorizes him to use the Alien Enemies Act of 1798, as a tool of mass detention and deportation. The Alien Enemies Act of 1798 is a component of the notorious Alien and Sedition Acts, the only part of that legislation that remains on the books today. The Alien Enemies Act allows detention and removal of migrants only when there “is a declared war between the United States and any foreign nation or government, or any invasion or predatory incursion is perpetrated, attempted, or threatened against the territory of the United States by any foreign nation or government.” In that event, the president is given the power to detain or remove “all natives, citizens, denizens, or subjects of the hostile nation or government, being of the age of fourteen years and upward, who shall be within the United States and not actually naturalized.” Notably, the Act allows detention and deportation even of legal immigrants, so long as they have not yet become naturalized citizens.
In previous litigation against the Biden administration, Texas’s GOP state government used the “invasion” argument to claim the state could adopt anti-immigration policies that might otherwise violate federal law, such as detaining and deporting undocumented immigrants whom federal law and policy allowed to remain in the United States, and placing buoys in the Rio Grande River. They relied on Article I, Section 10, Clause 3, of the Constitution, which states that “[n]o state shall, without the Consent of Congress, … engage in war, unless actually invaded, or in such imminent Danger as will not admit of delay.” Texas claimed illegal migration and drug smuggling qualify as an “invasion” and, accordingly, the Constitution empowers state military action even in the absence of congressional authorization for war.
These claims that illegal migration and drug smuggling amount to an “invasion” are contrary to the text and original meaning of the Constitution, and would have destructive implications if allowed to prevail.
The Founders were clear that the invasion clauses of the Constitution refer to organized armed attacks. As James Madison put it in his Report of 1800, “[i]nvasion is an operation of war.”
The constitutional text makes clear that invasion refers to an actual attack. The Guarantee Clause pairs “invasion” with “domestic Violence”—which in 18th century usage refers to uprisings against the state government (not the modern meaning denoting violence in intimate relationships). Under the long-standing legal doctrine of noscitur a sociis, “a word may be known by the company it keeps.” Here, it makes little sense to assume that “invasion” includes nonviolent actions such as illegal migration or smuggling, when it is coupled with “domestic Violence.”
The original meaning is consistent with the text. Legal scholar Frank Bowman notes that “throughout the Constitutional Convention and the state ratification debates that followed, delegates and commentators used the term ‘invasion’ over and over” in ways that, with rare exceptions for “metaphorical” uses, “invariably refer… to a hostile armed incursion into or against the territory of the states or the nation.”
The absurdity of Trump’s and Texas’s arguments becomes even more clear when we recall that the invasion provisions of the Constitution – especially the Guarantee Clause – were understood to protect not only against foreign invasion, but against invasions of one state by another. As Madison put it in the Virginia ratifying convention for the Constitution, “A republican government is to be guarantied to each state, and they are to be protected from invasion from other states, as well as from foreign powers.” During the Founding era and for years thereafter, some states restricted entry of people from other states, especially by free Blacks and “paupers.” But no one in the Founding era believed such movement could qualify as an “invasion,” entitling the receiving state to “engage in war” in response. The same goes for smuggling of contraband goods across state lines (it is common for states to ban some goods that are legal in neighboring states), or if people who might be carriers of contagious disease migrated from one state to another.
The Administration uses the involvement of cartels in cross-border drug trade to buttress their position. But the fact that some cross-border smuggling is conducted by organized crime does not make it the equivalent of an armed attack. Otherwise, the United States was constantly under “invasion” under Prohibition, when armed organized crime organizations smuggled illegal alcoholic beverages across the Canadian border. Smuggling by organized crime groups is a virtually inevitable consequence of prohibition regimes, whether for alcohol in the Prohibition era or drugs today. By this theory, there is a constant, never-ending state of “invasion.”
The threat of cross-border terrorism is another possible rationale for the administration’s invasion theory. But the actual incidence of such terrorism is negligible. As a study by my Cato Institute colleague Alex Nowrasteh shows, from 1975 to 2023, “[i]llegal-immigrant terrorists who crossed either a land or water border into the United States have killed or injured zero people in attacks on US soil.” That doesn’t necessarily prove the threat is zero. But if even a tiny risk of cross-border terrorism amounts to an “invasion,” then once again we have an absurd situation where an invasion exists at all times.
The Trump executive order also cites the danger of immigrants spreading contagious diseases as a possible justification for his assertion of power. The available evidence indicates that increased illegal migration does not actually increase contagious disease prevalence above base rates, as areas with more undocumented migrants do not have higher levels than those with fewer. Regardless, being a possible carrier of disease does not make one an “invader.” Otherwise, once again an “invasion” would exist at virtually all times, since almost every person crossing a border could potentially be such a carrier.
Perhaps the rationale for the Administration’s position is that a large amount of illegal migration or smuggling qualifies as “invasion,” even if a smaller amount does not. But illegal entries across the southern border are actually low right now, down to their lowest level since August 2020, when the rate was depressed by the economic crisis caused by the Covid pandemic. If the current rate qualifies as an “invasion,” there will, once again, virtually always be one. Moreover, the Trump order doesn’t establish any metric for determining how much illegal migration is enough to be an invasion. Rather, it claims that determination is up to the president’s discretion, once again ensuring the “invasion” rationale can be used at any time.
Trump’s and Texas’s invasion arguments have sweepingly dangerous implications. The Trump order asserts that the president has the right to use the invasion theory to shut down nearly all immigration for so long as he wants, even that otherwise authorized by federal statute. This is a blatant usurpation of congressional authority. In so far as the Administration’s invasion rationale is, as shown above, one that actually applies at all times, the trampling of separation of powers is even greater.
In addition, the invasion argument would give the federal government the power to suspend the writ of habeas corpus almost any time it wants, thereby enabling detention of people – including U.S. citizens — without trial or filing charges. The Suspension Clause of the Constitution states that “[t]he Privilege of the Writ of Habeas Corpus shall not be suspended, unless when in Cases of Rebellion or Invasion the public Safety may require it.” If illegal migration and drug smuggling qualify as “invasion” for purposes of triggering state and federal authority under the Invasion Clause of Article I and the Guarantee Clause of Article IV, they also surely qualify as such under the Suspension Clause. Since there is significant illegal migration and cross-border smuggling at virtually any time, this would be a blank check for suspending vital civil liberties.
The habeas corpus issue is another reason to doubt that illegal migration and smuggling qualify as “invasion” under the original meaning. British violations of the writ were one of the major grievances that led to the American Revolution. If the Guarantee Clause effectively gave the federal government authority to suspend the writ and engage in arbitrary detention any time the president wants, that would surely have been noted by contemporaries and raised as an issue in the fight over the ratification of the Constitution.
The potential use of the invasion theory to trigger the Alien Enemies Act of 1798 also poses severe dangers. As noted above, it would allow detention and deportation even of legal immigrants. While the Act was enacted a decade after the drafting of the Constitution, the meaning of “invasion” under that act is likely to be the same as in the Constitution. In addition, the Act can only be used if the “invasion” or “predatory incursion” (also a type of violent attack) is perpetrated by a “foreign nation or government.” Migration or smuggling by private parties does not qualify.
While it may not be a major issue over the next four years, letting states declare invasions over migration or smuggling also poses dangers. In addition to empowering them to flout federal law, the Constitution also authorizes them to “engage in war” in response. This creates a danger of a state government dragging the United States into a war with foreign nation, such as Mexico, without congressional authorization.
So far, courts have uniformly rejected the immigration/invasion argument in cases raised by Texas, and by a number of other states dating back to the 1990s. To my knowledge, there have not yet been legal challenges filed against Trump’s “invasion” order. But I hope and expect that will change. Even if some of his border policies get upheld by courts on statutory grounds, it is important to bury the dangerous invasion theory. Congress would also do well to assert its prerogatives and push back against this attempted usurpation of power by the executive, though there is little prospect the GOP-controlled Congress will do so in the near future.
While it is unlikely that courts will endorse the argument that migration and drug smuggling qualify as invasion, they might rule that the definition of “invasion” is “political question” that courts are not allowed to address. Several previous court decisions have held that the definition of “invasion” in the Constitution is a political question (thereby preventing state governments from invoking broad definitions of “invasion” in order to be able to “engage in war” without federal authorization), though they have also simultaneously held that an illegal migration does not qualify as “invasion” because an invasion requires a large-scale armed attack see pp. 20-22 of my amicus brief in one of the Texas cases).
For example, in 1996, the Second Circuit held that “In order for a state to be afforded the protections of the Invasion Clause, it must be exposed to armed hostility from another political entity, such as another state or foreign country that is intending to overthrow the state’s government,.” The Third, Fifth, and Ninth Circuits have all reached similar conclusions, most recently in one of the cases brought by Texas. No federal court has ever adopted a definition of “invasion” like that advocated by the Trump administration.
There is no good reason to hold that the definition of “invasion” is a political question, especially if doing so would give the president a blank check to usurp power over the Congress and suspend the writ of habeas corpus anytime he wants. Such a vast concentration of power would surely go against the original meaning, as it would enable the president to engage in arbitrary detention at will – exactly the kind of abuse early Americans had experienced at the hands of the British and sought to prevent in the future. “Invasion” has a clear definition readily susceptible to judicial interpretation. However, the “political question” doctrine is inherently nebulous, and it is not easy to foresee what courts might do with it in this instance.
Trump’s and Texas’ invocation of the idea that illegal migration qualifies as “invasion” is extremely dangerous, and at odds with the text and original meaning of the Constitution. Federal courts would do well to continue to reject this theory.
Editor’s note: This piece is part of the Collection: Just Security’s Coverage of the Trump Administration’s Executive Actions