Four Questions and Few Answers About the Invasion Clause

On inauguration day, President Trump signed a proclamation “guaranteeing the states protection against invasion.” Trump determined that the federal government had “failed in fulfilling [its] obligation to the States” at the southern border and would “take measures to fulfill its obligation to the States.” In 2024, Texas Governor Greg Abbott also declared that there was an invasion at the southern border. Most critics saw these actions as partisan statements with no actual legal effect. I disagree. There are important constitutional ramifications to declaring an invasion. If there is an invasion, both the federal government and the states receive additional war powers to repel that invasion. Federal laws that conflict with those war powers may give way. Moreover, the children of invaders may not be entitled to citizenship at birth. 

The concept of an invasion may seem obscure today, but it was certainly on the Framers' minds. Four separate clauses of the Constitution reference invasion. Some scholars and judges argue these issues are open-and-shut. They claim that an invasion must be hostile, peaceful migrants cannot be invaders, and the courts have the power to second-guess the president’s proclamation. I am not so sure. Here, I will address four questions with few definitive and settled answers. First, what is an invasion? Second, who can declare an invasion? Third, what happens to other federal laws during an invasion? And fourth, do the children of invaders receive birthright citizenship? Once again, Trump has brought long-forgotten provisions of the Constitution to the arena and forced our polity to grapple with whether this President should be treated any differently from all other presidents.

What is an invasion?

The Supreme Court has not defined what constitutes an invasion. Several lower courts have interpreted an “invasion” to mean an incursion by a foreign army or armed hostility. I would not put too much stock in these precedents, which breezily defined the term in only a few sentences. Originalist evidence, I think, cuts in both directions. Robert Natelson and Andrew Hyman contend that invaders do not need to be organized, armed, or part of a foreign army. They point out that during the founding era, “Pennsylvanians used [the] term [invaders] to describe the essentially peaceful immigration of Connecticut settlers into Pennsylvania’s Wyoming Valley, because the settlers were relying on legal title that the Pennsylvania government did not recognize.” They find similar definitions in over a dozen founding-era dictionaries and other eighteenth century sources.

Ilya Somin, by contrast, argues that peaceful migrants cannot be invaders. In Federalist No. 43, James Madison referred to an invasion as “foreign hostility.” And in the Report of 1800, Madison describes “invasion . . . as an operation of war.” Both of these quotes from Madison are helpful but not conclusive. Madison describes what might be viewed as the typical invasion, but he doesn’t say that an invasion can only be hostile.

President Trump’s order observed that “[o]ver the last 4 years, at least 8 million illegal aliens were encountered along the southern border of the United States.” In 1790, the population of the United States was approximately four million people. How would Madison view a wave of migrants, double the population of the United States, entering the country without authorization? Still, size probably shouldn’t matter. In Ex Parte Quirin (1942), the Supreme Court referred to eight Nazis who arrived on a New York beach as “enemy invaders.” In any event, a definitive originalist analysis should rely on more than scattered quotes. A corpus linguistics study of “invasion” would be helpful here.

There are other ways to determine original meaning. The Constitution juxtaposes invasion (an external threat) with insurrection, rebellion, or domestic violence (internal threats). This text suggests that invasion should be read in much the same fashion as an insurrection. But that analysis doesn’t settle the matter. For much of 2024, the nation was divided about whether January 6 was an insurrection and whether Donald Trump was an insurrectionist. (That issue, once apparently urgent, has fallen eerily quiet in the wake of Trump’s electoral victory.) Scholars and the Colorado Supreme Court concluded that the definition of an insurrection was easy to determine and that Trump’s conduct fell squarely in the definition of insurrection. The U.S. Supreme Court unanimously declined to disqualify Trump, though it did not reach the merits. In my research on the topic, I avoided opining on the precise definition of insurrection. I did so, in part, because there was little judicial precedent on the meaning of this fact-specific term, and applying it to an event as complex as January 6 seemed beyond the ordinary work of judges. I would approach invasion in much the same fashion.

Measuring the threat from the rioters on January 6 and determining whether those events rose to the level of an insurrection is a political judgment call. Likewise, determining the harm that may flow from migration at the southern border and whether it rises to the level of an invasion is a political judgment call. I don’t think the invasion question can be reduced to neat legal tests that judges and law professors like to make up. In a related context, the courts have uniformly found that a declaration of war is a nonjusticiable political question due to the lack of “a coherent test for judges to apply to the question what constitutes war.” I think the same principle should apply to invasion. 

The more relevant question is which political actors can establish that there is an invasion. On the campaign trail, Trump repeatedly referred to the situation at the border as an invasion, and voters went to the polls with full knowledge of that fact. What happened in the administration’s first month should not be a surprise.

Who can declare an invasion?

The Constitution is quite clear that only Congress can declare an offensive war. However, the text is not so clear about who can declare an invasion. This power seems to be spread throughout all branches of the government, except the judiciary. Four provisions of the Constitution refer to invasions. First, Congress can suspend the “Privilege of the Writ of Habeas Corpus . . .  in Cases of Rebellion or Invasion the public Safety may require it.” It is long settled that the President lacks the power to unilaterally suspend the writ, and detain prisoners without any judicial process, even during war. Thus, Congress must have the power to declare when an invasion occurs. But that is not the only path. 

Second, Congress has the power “[t]o provide for calling forth the Militia to . . . suppress Insurrections and repel Invasions.” But the President, as Commander in Chief of the Militia, has the power to decide when there is an insurrection or invasion, to call forth the Militia. This power is expressly recognized in the Militia Act of 1795. And in Martin v. Mott (1827), Justice Joseph Story wrote that this power “is confided to the Executive of the Union.”

Third, the United States must “protect each [state] against Invasion.” This text does not specify which branch of the federal government makes such a “guarantee.” In Luther v. Borden (1849), the Supreme Court wrote this sort of power is “given to the President.” The Court cited the need for the executive to move with dispatch during an “exigency.” Presumably, Congress could also declare an invasion, and I do not think that action would be subject to a filibuster. But what happens if the federal government fails this guarantee? 

Fourth, a state can “engage in war” if it is “in such imminent Danger as will not admit of delay” to seek congressional consent. But a state does not need to consult Congress when it is “actually invaded.” As Judge James C. Ho recently observed, a state can “engage in war” to stop an actual invasion immediately without “such temporal restrictions.” Judge Ho added that in 1874, Texas Governor Richard Coke declared an invasion of Texas and that the state had “the same ‘right to self-defense that would ‘ordinarily reside in the United States.’” In that case, Congress did not consent to the declaration of an invasion, though the executive branch acquiesced.

The Constitution affords Congress, the president, and the states the power to declare an invasion—every branch except the judiciary. This evidence is all the more reason for courts to resist the urge to define an invasion and adjudicate whether the complex situation at the southern border is an invasion.

What happens to other federal laws during an invasion?

The Constitution assigns certain federal functions to the states. For example, state legislatures have the power to regulate elections, provide for the appointment of presidential electors, and ratify constitutional amendments. Other federal functions are assigned to the state executive branch. If there is a House vacancy, “the Executive Authority” of that state schedules an election “to fill such Vacancies.” And if there is a Senate vacancy, the Executive may “make temporary Appointments.” The Constitution also assigns a federal function to multiple branches of a state government. In cases of “domestic Violence,” a petition to the United States for protection can be made by the state legislature or the state executive “when the Legislature cannot be convened.”

When the Constitution assigns a federal function solely to a state, those actions authorized by the Constitution are the supreme law of the land. Congress cannot regulate those federal functions by statute. The declaration of an invasion is also a federal function. The President has the power to declare an invasion and protect the states from an invasion. States can declare an invasion and use war powers when “actually invaded.” 

With good reason, critics are concerned about this Invasion Clause argument. If the President or the states can exercise a federal function, the Constitution directly authorizes those actions. Under the Supremacy Clause, these constitutional functions would likely prevail over any contrary statutory authority. Those actions would only be prohibited if they conflict with another constitutional guarantee, such as the requirement to afford people with the due process of law. For example, only Congress can suspend the constitutional guarantee of habeas corpus in case of invasion. Neither the President nor the states can take this step. But in case of an invasion, where the President or states are acting pursuant to their respective invasion clauses, the Immigration and Nationality Act might give way. In 2024, Texas Governor Greg Abbott used a declaration of an invasion to support placing buoys in the Rio Grande River to prevent migrants from crossing into Texas, even though federal environmental law may otherwise prohibit that action.  

I fully concede this is a startling argument in its breadth. But that conclusion is a direct consequence of the federal function that the Constitution jointly assigns to the federal government and the states. A common refrain is that this argument cannot be right because the President or State could declare an invasion anytime they wanted, thus avoiding federal authority. I think this argument downplays the fact that millions of migrants are crossing the border over many years. If this flow were stopped or heavily reduced, it would be harder to justify an invasion. Even if the courts could not intercede, political pressure can prevail. In other words, the fact that it is so easy today to say there is an invasion is a direct byproduct of failure to address this flow of migrants over the decades. Under this argument, the federal government disregarded its guarantee to the states to stop an invasion, which may justify unusual efforts now. And if Congress isn’t happy with the actions being taken by the state and wants to remove the state's war powers, there is a straightforward option: stop the invasion

Some critics may argue that granting the federal government and the states such a sweeping power to override statutory law is itself a reason to reject such a broad reading of the Invasion Clause. Or stated differently, the fact that prior presidents have not declared an invasion–at least apart from asking Congress to declare war–suggests that the invasion power is not a trump card to federal immigration law. This argument only goes so far. If there is an ongoing invasion, and Presidents have failed to address this crisis for decades, that would make the state’s prerogative to act all the more urgent. 

Do the children of invaders receive birthright citizenship?

I have long taken the position, as a matter of original meaning, that the children of illegal aliens receive birthright citizenship. A couple not lawfully in the country, would still be subject to our jurisdiction, and their child would be a citizen by birth. The Supreme Court’s decision in United States v. Wong Kim Ark (1898) is often cited as definitive proof of this principle. But as Richard Epstein pointed out in these pages, that case concerned a child born to lawful permanent aliens in the United States. The Supreme Court could extend the holding of Wong Kim Ark to protect the children of illegal aliens, and I think such an extension would be justified by original meaning, but that step has not happened yet.

Though my thinking on the citizenship clause of the Fourteenth Amendment has not changed, recent developments concerning the Invasion Clause have raised new questions. In November, I interviewed Judge Ho. He pointed out that “birthright citizenship obviously doesn't apply in case of war or invasion” and “No one to my knowledge has ever argued that the children of invading aliens are entitled to birthright citizenship.”  Ho is right. 

Wong Kim Ark observed that the “children of aliens within territory in hostile occupation” would not be subject to the jurisdiction of “the sovereign whose domains are invaded.” As a general matter, a successful invasion can lead to a long-term occupation. And the children of those invaders and occupiers would not be birthright citizens. Recently, a Maryland district court recognized that the children of occupiers would not be birthright citizens, but said “[t]his exception to citizenship by birth plainly does not apply to the children described” in Trump’s order. That distinction between children of occupiers and children of invaders does not hold up.

Modern scholars seem to agree with this conclusion. A law professor from Northwestern wrote that invaders “deny[] and destroy[] the jurisdiction of the United States—which is why the government is allowed to shoot them without due process.” Another law professor from Berkeley concluded that if a child’s “parents are in the U.S. as an invading army,” she would not be a birthright citizen. This point should not be controversial. Of course, these scholars maintain that an invasion must be hostile. But if they’re wrong on that point, and invaders can be non-hostile, the children of two peaceful invaders would still not be birthright citizens. I’m not sure what would happen to a child with one invader parent, and one non-invader parent; for example, what would happen if Mohamed Atta fathered a child with a U.S. citizen while planning the 9/11 attacks?

Again, much turns on the first three questions posed in this essay. If the situation on the southern border is an invasion, and the President can declare there is an invasion, then federal statutes governing citizenship, and indeed the Citizenship Clause of the Fourteenth Amendment itself, may give way. I don’t pretend any of this is clear, but careful work must be done to walk through each issue.

Ultimately, I am certain about one principle, which I repeated through Trump’s first term, and I will repeat for four more years. Whatever the meaning of the invasion clause, the answer cannot turn on the unprecedented nature of the Trump presidency. These legal questions should have an answer that does not depend on Trump. Either the President has the power to declare an invasion, or he does not. Either the state has the power to declare an invasion, or it does not. Either the judiciary has the power to second-guess a federal declaration of an invasion, or it does not. Either the children of invaders receive birthright citizenship, or they do not. The answer to those questions has to be the same in 1788, 1868, or 2025. I’ve raised four questions here, and I do not think many clear and settled answers exist. But in any event, these issues cannot decisively cut against the position taken by President Trump and Texas.

Josh Blackman holds the Centennial Chair of Constitutional Law at the South Texas College of Law Houston, and is the co-author of An Introduction to Constitutional Law: 100+ Supreme Court Cases Everyone Should Know. He is a contributing editor to Civitas Outlook.