While the State of Washington sues President Trump over refugee visas, we will have a weather eye out for the part of the Constitution known as Article IV, Section 4. It is a sui generis part of our national parchment. Much of the document is concerned with granting, enumerating, and separating powers for things the government can do. Much of the Constitution is given over to listing what the government, and in some instances, the states, can not do. It is only in Article IV, Section 4 that the Constitution sets down things that the United States must do.

These are sometimes called the obligations clauses. The first says that the United States “shall guarantee to every state in this Union a Republican Form of Government.” The wording doesn’t denote the Republican Party (which didn’t exist at the time the Constitution was framed) but rather representative government, as opposed to, say, a monarchy. This is followed by a clause that says that the United States “shall protect” each of the states “against Invasion.” The government has no choice in the matter. It shall protect the states against invasion.

In its plain language, the clause raises a startling possibility. Could it be that President Trump is not merely permitted to impose the travel restrictions that have landed him in court but is actually required to pursue them? Could he argue that the countless threats from the Islamic State and al-Qaeda to secret their agents among the refugees fleeing the war zones constitute evidence that an invasion is underway? Could he argue that the constitutional oath that he has just sworn obligates him to undertake extreme vetting to protect the states against invasion by agents of the Islamic State and al-Qaeada?

It’s not our purpose here to say that America is being invaded. Neither would we want to deny that America is being invaded. We might well be unable to verify whether we’ve been invaded until violence erupts in our places of work, nightclubs, and houses of worship. America — never mind, among others, France and Germany — has seen enough of the tactics of the Islamic State and Al-Qaeada to know their penchant not only for terror but for surprise. In any event, who, under the Constitution, gets to define an invasion and determine that Article IV Section 4 needs to be invoked?

The Constitution itself is silent on what constitutes an invasion. We know that the Founders were at least partly concerned with the an invasion of one of our own states by forces of another. Surely, though, the same obligation to protect exists in respect of foreign invasions. So who gets to decide this question? The Congress? The Courts? The President? How about the Reverend Clergy or the Press? It strikes us as at least logical — and constitutional — that in such a question the president, who operates our spy services and is commander-in-chief, counts for something.

So far, federal filings in the case of Washington v. Donald Trump offer no hint that attorneys for America are going to seek to invoke Article IV, Section 4. The constitutional sages from whom we’ve sought informal advice smile indulgently but suggest that no one can ever remember an Article IV, Section 4 action. The last time a state talked of calling on the guarantee clause was the Dorr War in Rhode Island (President Tyler demurred).

The invasions clause, not so much. The Heritage Foundation’s wonderful guide to the Constitution quotes St. George Tucker, one of our greatest judges, as suggesting that the clause guarded against “the possibility of an undue partiality in the federal government in affording it’s [sic] protection to one part of the union in preference to another, which may be invaded at the same time.” In any event, by Heritage’s tally, the clause has never been invoked.

Then again, too, there’s always a first time. If it be said that an invocation of this clause on the current state of the crisis would stretch the meaning of invasion, one could ask: Who wants to carp about not-previously-asserted interpretations of the Constitution? Abortion and birth control were protected in this country partly on the citation of the Third Amendment, which prohibits quartering troops in private homes during peacetime. By that example, for Mr. Trump to cite Article IV, Section 4 would look prosaic.