The Volokh Conspiracy
Mostly law professors | Sometimes contrarian | Often libertarian | Always independent
It has become commonplace for right-wing activists and GOP politicians to describe illegal migration across the southern border as an "invasion." While much of this is just political rhetoric, some have argued that the Guarantee Clause of the Constitution, gives state and federal governments the authority to use military force to block such migration. The Clause states that "The United States shall guarantee to every State in this Union a Republican Form of Government, and shall protect each of them against Invasion; and on Application of the Legislature, or of the Executive (when the Legislature cannot be convened) against domestic Violence." Then-Arizona Attorney General Mark Brnovich advanced this theory as a justification for military action in a February 2022 opinion. Andrew Hyman recently made a similar argument at the Originalism Blog.
As a matter of logic and common sense, the equation of illegal migration and invasion makes little sense. Invasion involves large-scale use of force (or at least threat of force) to seize territory. Russia's attack on Ukraine is an invasion. Migrants crossing a border in search of freedom and opportunity are not. We do, of course, have metaphorical uses of the word "invasion," as with "invasion of privacy" or even the 1960s "British Invasion" of UK rock music. But such metaphorical uses should not be conflated with literal ones. Claims that immigration is a kind of "invasion" are about the latter.
The text of the Guarantee Clause suggests that it refers to violent attack. "Invasion" is paired with "domestic Violence" (which here obviously means uprisings against the state government, not the modern use of the term to denote violence in family and intimate relationships). Hyman also cites the provision of the Constitution indicating that "No State shall, without the Consent of Congress … engage in War, unless actually invaded, or in such imminent Danger as will not admit of delay." But the "invasion" referred to here is pretty obviously an armed attack. Otherwise, it would not make sense to "engage in War" as a response to it. I think it obvious that the "war" referred to here is a literal war against a foreign power, not a metaphorical war, such as the War on Drugs or the War on Poverty. This Clause is an exception to the constitutional requirement that only Congress has the power to declare war.
Brnovich and Hyman both cite James Madison in support of their position. But they overlook the one time he discussed the Guarantee Clause's reference to "invasion" specifically in the context of immigration. He did so in his Report of 1800, which rebutted claims that the Alien Friends Act of 1798 (which gave the president broad power to expel non-citizens) was authorized by the clause:
It is said, that Congress, are, by the constitution, to protect each state against invasion; and that the means of preventing invasion, are included in the power of protection against it….
Invasion is an operation of war. To protect against invasion is an exercise of the power of war. A power therefore not incident to war, cannot be incident to a particular modification of war. And as the removal of alien friends [citizens of countries with which the US is not at war] has appeared to be no incident to a general state of war, it cannot be incident to a partial state, or a particular modification of war.
Nor can it ever be granted, that a power to act on a case when it actually occurs, includes a power over all the means that may tend to prevent the occurrence of the case. Such a latitude of construction would render unavailing, every practicable definition of particular and limited powers. Under the idea of preventing war in general, as well as invasion in particular, not only an indiscriminate removal of all aliens, might be enforced; but a thousand other things still more remote from the operations and precautions appurtenant to war, might take place. A bigoted or tyrannical nation might threaten us with war, unless certain religious or political regulations were adopted by us; yet it never could be inferred, if the regulations which would prevent war, were such as Congress had otherwise no power to make, that the power to make them would grow out of the purpose they were to answer. Congress have power to suppress insurrections, yet it would not be allowed to follow, that they might employ all the means tending to prevent them…
As Madison put it, "invasion is an operation of war." Immigration, by contrast, is not. And, as he explained, if this power is interpreted broadly to include anything that might reduce the risk of war or increase national security at the margin, it would undermine virtually all structural limits on federal power. It would also render many of the federal government's other enumerated powers superfluous. For example, there would be no need for the power to "raise and support," armies, the power to impose taxes (which could help repel invasions!), and many other powers listed in Articles I and II of the Constitution.
While overlooking the Report of 1800, Brnovich and Hyman both cite Madison's statement about the invasion provision of the Guarantee Clause in Federalist 43:
A protection against invasion is due from every society to the parts composing it. The latitude of the expression here used seems to secure each State, not only against foreign hostility, but against ambitious or vindictive enterprises of its more powerful neighbors. The history, both of ancient and modern confederacies, proves that the weaker members of the union ought not to be insensible to the policy of this article.
Brnovich and Hyman claim that the reference to "ambitious or vindictive enterprises of its more powerful neighbors" suggests that "invasion" includes nonviolent actions. But the point of this phrase is pretty obviously not to imply a broad definition of "invasion" but to emphasize that the Guarantee Clause protects states against invasions by other states (the "more powerful neighbors") as well as foreign powers ("foreign hostility"). The reference to the history of "confederacies" and the needs of the "weaker members of the union" reinforces this interpretation.
The possibility of warfare between states was more than just a hypothetical possibility in the Founding era. Conflicts between them had occurred during the colonial era, most notably the long-running, occasionally violent, dispute between New Hampshire and New York over possession of the territory that eventually became the state of Vermont.
Brnovich also cites Madison's comments, in the Virginia debate over ratification of the Constitution, to the effect that "[t]he militia ought to be called forth to suppress smugglers." But this passage has nothing to do with the power to protect against invasions. Rather, it was a response to Patrick Henry's attack on Congress' power of "calling forth the Militia to execute the Laws of the Union." Madison does not claim here that smuggling qualifies as an invasion. And using the militia to enforce laws isn't the same thing as taking the kind of large-scale military action that might be justified in the event of a genuine invasion.
If you want to know what Madison thought about the claim that immigration counts as "invasion," look to the Report of 1800, where he actually discusses that issue.
Hyman also tries to buttress his case with a clever, but unpersuasive hypothetical: "It would certainly be odd if the forces of a foreign government could simply leave their weapons at home, and thereby deprive a U.S. state of power to resist their incursion." If these foreign troops left their weapons at home, their "invasion" wouldn't be much of a threat! I think it may not even be worthy of the name. But if an attack by unarmed troops did qualify as an invasion, it would be because they are still part of an organized military force trying (even if not very effectively) to use violence to seize territory. The difference between that and illegal immigration is pretty obvious. The same reasoning applies to an attack by non-state armed forces, such as terrorists.
It might be argued that any illegal movement from one place to another qualifies as an "invasion." By that standard, however, an invasion occurs anytime someone smuggles in contraband, violates tariff regulations, and so on. In the pre-Civil War era, some states, such as Indiana, enacted laws banning the in-migration of free blacks from other states. But it would be absurd to claim that black migrants who violated these laws were thereby "invading."
Similarly, one can argue that an "invasion" occurs anytime at least some migrants engage in violence (as in the case of drug cartels operating in the border area, for example). But by that standard, one state has "invaded" another anytime criminals cross a state border to engage in any violent action. A real "invasion" requires a large-scale attack on the territorial or political authority of the state. Small-scale, nonpolitical private violence doesn't qualify. The latter can, of course, still be dealt with by normal law enforcement actions. But it doesn't justify large-scale use of military force of the kind that can be used to respond to an invasion.
Finally, it's worth noting that even if the Guarantee Clause gives the federal government power over immigration, it does not give states any authority of any kind (except to call for federal assistance in suppression "domestic Violence"). The power to protect against "invasion" is given only to "the United States," which is the way the Constitution refers to the federal government.
At least for the moment, the practical legal significance of the "invasion" issue is very limited. In the Chinese Exclusion Case (1889), the Supreme Court held (very wrongly, in my view) that the federal government has a nearly unlimited "inherent" power to restrict immigration, without relying on the Guarantee Clause to reach that conclusion. For their part, state governments have fairly broad authority to use the National Guard or other military forces in a law enforcement role, if they so choose. The main limit on that, if any, lies in their state constitutions. For these and other reasons, we are unlikely to see any significant litigation over the meaning of "invasion" as it relates to immigration, anytime soon.
But the rhetoric of "invasion" and citations to constitutional provisions on that subject do serve the purpose drawing a false moral and legal equivalency between migrants and invaders. They thereby potentially justify harsher and more militaristic responses to migration than might otherwise be contemplated. Moreover, the Guarantee Clause argument could potentially become legally significant when and if there is broader recognition of the many weaknesses of other constitutional rationales for federal-government migration restrictions. For these reasons, I thought it worthwhile to critique the invasion theory, even though it's unlikely to have much immediate legal effect.
Hyman also cites the Migration or Importation Clause and the Define and Punish Clause as possible constitutional justifications for federal power over immigration (though not necessarily for the use of military force). I addressed the former theory here, and the latter here.