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Constitutional Theory and the Meaning of "Invasion"

Leading constitutional law scholars Larry Solum and Mark Tushnet opine on how we might answer this question.


What a real invasion looks like. Russian armored vehicle in Ukraine. March 2022. (NA)


Some conservatives have argued that illegal migration and drug smuggling across the southern border qualify as "invasion" under the Constitution. This issue is currently being litigated in two cases before the US Court of Appeals for the Fifth Circuit. Two leading constitutional theorists—Larry Solum (University of Virginia) and Mark Tushnet (Harvard)—have recently written posts outlining how constitutional theory could be used to address the meaning of "invasion."  Here's an excerpt from Tushnet's post:

What are we to make of the term "invasion," which occurs three times in the Constitution (in the habeas-suspension clause, in the Compact Clause [as "actually invaded"], and in Article IV)? The term has its place in contemporary conservative discourse, which characterizes what's happening at the US southern border as an invasion. One can imagine a Trump administration suspending habeas in connection with those who cross the border without authorization. Conservatives might assert that Article IV places a duty on the United States to protect states against invasions (one of which is occurring) and that the President's failure to do so provides the basis for impeaching him for failing to take care that the laws be faithfully executed. (At least one impeachment resolution invokes this theory.)

Is this an example of (a) impermissible linguistic drift or (b) permissible specification of vague constitutional terms within the bounds of reasonable interpretive flexibility? I did some quick and dirty research (this is a blog post, after all), and came up with this. The 1785 edition of Samuel Johnson's dictionary defines "invasion" as "a hostile entrance upon the rights or possessions of another," and provides four illustrations, of which two involve invasions by organized military forces of hostile nations (and the other two of which seem to me metaphorical). Noah Webster's 1828 dictionary's first definition is: "a hostile entrance into the possessions of another; particularly, the entrance of a hostile army into a country for the purpose of conquest or plunder, or the attack of a military force. The north of England and south of Scotland were for centuries subject to invasion each from the other. The invasion of England by William the Norman was in 1066."

My real puzzle isn't about "the" answer to the question posed in the preceding paragraph. (About 15 years ago I suggested, in passing, that the September 11 attacks could plausibly be characterized as an invasion for purposes of habeas suspension, thus assuming that organized attacks by a hostile non-state actor could count as an invasion. What about the ISIS-influenced attack by a single individual at Fort Hood years later?) The puzzle is about how to think about figuring out the answer.

I actually don't think the issue here is particularly difficult. Both at the time of the Founding and today, "invasion" usually means an organized armed attack, but also has secondary meanings, many of which are more metaphorical. Which one is relevant in a given case depends on the situation.

The use of "invasion" in the Constitution is an example of how the meaning of a potentially ambiguous word becomes clear in context. In other situations, "invasion" can sometimes mean a mere intrusion on rights (e.g.—"invasion of privacy"), or even just a metaphorical conflict, like the 1960s "British Invasion" of UK rock bands coming to perform in the US.

In the context of giving states the right to "engage in war" in response (which the Constitution authorizes a state to do in the event it is "actually invaded"), suspending the writ of habeas corpus (which the federal government can do if there is an "invasion"), and other relevant features of the Constitution,  it is limited to organized armed attacks.  Founding-era evidence supports this position. For more detail see my Lawfare article on this subject, and the amicus brief I filed in one of the Fifth Circuit cases on behalf of the Cato Institute and myself.

Solum's post is more extensive and detailed, and cannot easily be summarized. Anyone interested in this topic should read the whole thing! Here, I will only note that Solum emphasizes that, from an originalist perspective, "we would not want to focus solely on the word 'invasion.' Instead, we would aim to determine the meaning of whole clauses and articles in the context."

I agree completely! It is the context, particularly the wording of the clauses where the word appears, that ultimately determines the meaning of "invasion" in the Constitution. And the context makes clear that it is limited to organized armed attacks, and does not cover illegal migration, drug smuggling, and the like.

I think this context is also decisive from the standpoint of living constitutionalism. No plausible living constitutionalist theory would allow states to start a war over illegal migration or drug smuggling, without the authorization of the federal government. Nor would it give the federal government a blank check to suspend the writ of habeas corpus any time such things happen. As noted in my article and amicus brief, the latter power would not be limited to detaining undocumented migrants, but would cover US citizens and legal residents, as well.

I am planning to write an academic article on the meaning of "invasion," where I will address these issues in greater detail.

UPDATE: I initially neglected to include a link to Larry Solum's post. That error has now been corrected.