U.S. officials also arguably bent or broke several principles of international law.

The first and most relevant international law in the case of people seeking asylum at the U.S. southern border is the 1967 Protocol to the Refugee Convention, which the U.S. ratified in 1968. Under the “supremacy clause” of the Constitution, once it ratifies such a treaty, the U.S. is bound by it.

Under the 1967 refugee protocol, people who claim refugee status who reach the border of a country that is party to the treaty — or who even enter that country — are allowed to apply for asylum and to have their claims judged. These people may or may not get to stay, but the nation in question, the U.S. in this case, is obliged to give them a hearing.

Some of the barriers the U.S. has placed between asylum seekers and the relevant legal process — the waiting lists, the threat of detention or family separation and the presence of intimidating soldiers — might be legal, in a narrow sense. But they violate the spirit and the letter of the protocol. They are immoral.

In its use of force and its positioning of the military near the border, the Trump administration is clearly not treating this caravan as if it were composed of people seeking asylum. The administration is acting, in a kind of “Alice in Wonderland” way, where we go down the rabbit hole and enter an alternate reality, as if the United States were at war with these refugees, some of whom are wearing flip-flops.

But even if the U.S. were at war, U.S. behavior toward the asylum seekers would still be unlawful. The marchers posed no imminent threat to the armed agents. Scaling a wall to apply for asylum is not a violent attack.

The administration appears to be framing its response to the asylum seekers as a preventive strike on a hostile force. The president has said that the asylum seekers might, at some point in the future, commit crimes in the U.S., or that they might, one day, take American jobs. We all, President Trump suggests, should be afraid of what might happen. Under this logic, the administration may conceivably be thinking that it is preventing future problems by dealing harshly with people it judges to be potential threats to the security of the United States. Therefore, in this view, preventive force is justified.

Unfortunately for the administration, preventive strikes are generally forbidden under international law; even if you go some way toward accepting the government’s false premise, the actions are unjustified.

In 1841, U.S. Secretary of State Daniel Webster articulated important and now widely accepted international principles governing the use of military force to prevent a potential future threat.

The use of preventive force, Webster argued, is justified only in response to an imminent threat. As Webster put it, there must be “a necessity of self-defense, instant, overwhelming, leaving no choice of means, and no moment of deliberation.” And, he added, deadly force could be used only after nonlethal measures failed.

Such limits on preventive and preemptive attacks are understandable. Otherwise, fear of what might happen could be used to justify the use of force to respond to all sorts of threats, however distant or speculative. In the case involving the California-Tijuana border, there was time to deliberate on alternative, less potentially harmful ways of controlling these unarmed people. The people now on the border have been walking to the U.S. border for months, and many have been peacefully waiting there for weeks.

Trump has argued that U.S. border agents were acting in self-defense (“They were being rushed by some very tough people.”) That stretches the concept of self-defense to the point of meaninglessness.

Clearly, nothing the marchers have done poses an imminent threat to American lives. Further, despite all the aspersions the administration has cast on the asylum seekers, we have absolutely no way of knowing if any of them will commit crimes in the United States. (And again, applying for asylum is not a crime.)

Even in those cases in which a preemptive strike is justified — if, say, an armed group were massing near our border, planning an assault — the U.S. response must focus tightly on the immediate threat and must distinguish between the armed and unarmed. And any use of force must be proportionate to the actual threat.

Trump has suggested that border officials (or the military) would be justified in opening fire on anyone who throws rocks at them from the other side of the border. But firing live ammunition or even “rubber bullets” would surely be a disproportionate response to such a feeble attack.

Firing tear gas at unarmed civilians, including children, moreover, may also violate the 1949 Geneva Conventions’ principle of “distinction” — that is, distinguishing between combatants and noncombatants. Unarmed civilians are “in all circumstances” to be “treated humanely without any adverse distinction founded on race, color, religion or faith, sex, birth or wealth, or any other similar criteria.” The taking of hostages, and “outrages upon their personal dignity, in particular, humiliating and degrading treatment” is also prohibited. In short, even if a hot war existed at the Mexican border, the asylum seekers deserve the protection that should be afforded to all civilians under international humanitarian law.

Even when we accept the Trump administration’s stated and implied rationales for its conduct at the border, its conduct is indefensible. Yet when we argue on those terms, we have gone down an absurd rabbit hole. The U.S. is most obviously not at war with Mexico. Nor do the asylum seekers constitute an invading force.