In the Trump administration’s four years of undermining America’s image of decency, perhaps no policy did so as effectively, or as viciously, as his family separation policy – which separated 5,000 children, some as young as four months old, from their mothers and fathers.

The theory behind the policy was that inflicting excruciating pain on thousands of parents and children separated at the border would deter migration to the US. It was another example of the Trump administration’s calculated cruelty.

We now know something about why officials throughout the government went along with the family separation policy. They “were under orders from Trump”, Kevin McAleenan, the Department of Homeland Security’s commissioner of Customs and Border Protection, told Caitlin Dickerson of the Atlantic. McAleenan was “just following directions”, as Dickerson puts it. Those directions came from Stephen Miller, Trump’s fiercely anti-immigrant enforcer.

Just following orders. We’ve heard that before from perpetrators of great wrongs.

Whatever their reasons, the actions government officials took in pursuit of the family separation policy demand a response. Doing justice for the victims of the policy demands accountability for those who designed and implemented it. And deterring such conduct in the future is only possible if there are consequences for engaging in it.

International law offers a framework for accomplishing those goals and for seeing the family separation policy for what it was: a crime against humanity.

But before exploring that framework, let’s examine what we know about why government officials would go along with Trump and Miller’s calculated cruelty.

In 1963, the Yale psychologist Stanley Milgram offered the best-known answer. Milgram enlisted subjects in a “learning experiment”. Their job was to apply what they thought were increasing levels of electrical shock to “learners” whenever they gave incorrect answers.

Unknown to subjects, the “learners” were Milgram’s collaborators. They intentionally gave wrong answers and feigned excruciating pain as the voltage seemingly increased to severe shock. Under the direction of a “research administrator”, who became increasingly firm when subjects hesitated to apply more pain, two-thirds of them ended up administering the maximum dose of “electricity”.

As Milgram put it: “The extreme willingness of adults to go to almost any lengths on the command of an authority constitutes the chief finding of the study.”

Evil, it turned out, was as banal as Hannah Arendt, the famed political theorist, described it in her celebrated chronicle, Eichmann in Jerusalem. This is the evil done by those without whose complicity Trump’s family separation policy could not have been carried out.

Eichmann’s 1961 conviction, and those at Nuremberg, established the principle that individuals who claimed to be “just following orders” are as culpable for crimes they commit as those who give the orders.

And the 1998 “Rome statute” created a forum that can provide accountability for the people who designed and implemented the family separation policy – the international criminal court.

The Rome statute authorized the ICC to prosecute individuals who commit crimes against humanity, including “inhumane acts … [that] intentionally caus[e] great suffering, or serious injury to body or to mental or physical health.”

There is no question that systematic actions separating parents from children meet that definition.

While the United States is one of only seven countries not to have ratified the Rome Statute, this fact should offer little solace to those who violate its principles. Here is why.

First, under the “principle of complementarity”, the ICC may exercise its jurisdiction when a country is either unwilling or unable to investigate and prosecute crimes within its territory.

Applying the complementarity doctrine, in 2011 the ICC initiated prosecution of Libya’s one-time dictator Muammar Gaddafi, though his country never ratified the Rome statute.

Second, the Nuremberg principles that the United States wrote before the trials began justify prosecuting crimes against humanity in the complete absence of any agreement by an accused violators’ country. That Germany did not ratify those principles was no barrier to prosecution of Nazi officials at Nuremberg.

Third, the US has signed other international agreements incorporating protections against crimes such as the ones implicated by Trump’s policy to separate families. For example, in 1992, President George HW Bush signed the International Covenant on Civil and Political Rights (ICCPR), which Congress had ratified, making it the law of the land.

Article 24 of the ICCPR provides that “[e]very child shall have, without any discrimination as to race, … national or social origin … the right to such measures of protection as are required by his status as a minor.” As the UN high commissioner for human rights emphasized in a 2010 report, “the principal normative standards of child protection are equally applicable to migrant children and children implicated in the process of migration.”

Another relevant treaty under which American officials could be charged is the Convention against Torture and Other Cruel, Inhuman or Degrading Treatment or Punishment, ratified by the US in 1988. It defines “torture” as “any act by which severe pain or suffering, whether physical or mental, is intentionally inflicted … for such purposes as … punishing him for an act he or a third person has committed … when such pain or suffering is inflicted … at the instigation of a public official.”

While the Biden administration has made considerable progress reuniting families, it has not moved quickly enough to completely end the policy. It is up to the public to ensure that result and to demand that Trump administration officials answer for making crimes against humanity a centerpiece of US immigration policy.

There is more than enough binding law and precedent for bringing charges against those officials. They should have their day in court, where they can offer their legal defenses and explain to the world why they did what they did.

Prosecutors at The Hague should bring before the bar of justice Trump officials who instituted the policy of separating children from their mothers and fathers. Humanity and history require it.

• Austin Sarat is a professor of jurisprudence and political science at Amherst College and the author of Lethal Injection and the False Promise of Humane Execution

• Dennis Aftergut, a former federal prosecutor, is of counsel to Lawyers Defending American Democracy