When Robert S. Mueller III, the former special counsel, concluded in his final report in March and thereafter that a sitting president of the United States, and thus Donald J. Trump, is immune from criminal prosecution, he based his view primarily upon the legal analyses of two memoranda authored in 1973 and 2000 by the Department of Justice’s Office of the Legal Counsel (OLC).

But Mr. Mueller, like the OLC lawyers, ignored the worst-case scenario. The OLC memoranda never addressed immunity for the most serious possible crimes. Instead, the OLC addressed comparatively lesser (though important) offenses and the interference that criminal proceedings could subject a president to in the performance of his duties. What the United States cannot afford is to undermine the global rule that sitting heads of state have no immunity from prosecution for atrocity crimes. As we as a nation continue to work through the constitutional processes for addressing presidential wrongdoing, this important dimension should not be lost.

The OLC lawyers viewed the Constitution antiseptically, as if no possible calamity unleashed by a president could trigger immediate criminal indictment. The best that could be surmised is that presidential involvement in committing the most unimaginable of crimes would lead to impeachment, but even that sadly cannot be certain anymore given what we’ve learned from this political climate.

Yet, what if a president planned or ordered genocide, crimes against humanity, or the most devastating of war crimes? These are atrocity crimes of the worst character, often leading to the deaths and injury of thousands, sometimes hundreds of thousands, of innocent individuals, and the devastation of whole societies. Such atrocity crimes in recent decades ravaged, for example, Afghanistan, Bosnia-Herzegovina, Cambodia, central Africa, Chechnya, Croatia, Iraq, Myanmar, North Korea, Rwanda, Sierra Leone, Syria, and Timor-Leste.

A presidential order to unleash a systematic torture program, or a president’s intentional complicity in its implementation, might trigger an atrocity crime investigation. The U.S. Government should be resilient enough to bear both the responsibility and the consequences of any such investigation.

None of the cases examined in the OLC memoranda envisaged atrocity crimes. Is the OLC, and by extension, Mr. Mueller, plausibly suggesting that, in the commission of atrocity crimes, the only way to deal with the leader of the world’s most powerful nation, one that prides itself as the beacon of democracy and rule of law, is by impeachment? Is that the only real-time remedy, even though a president’s political party shields him by acquiescing in his deeds, with the option to prosecute only after a president has left office when a victorious party may want to move on and bury the past?

Imagine trying to explain the “not while president” position to any of our allies, most of which, unlike the United States, joined the International Criminal Court and waived any rationale to immunize top leaders from accountability and justice for atrocity crimes. Other international war crimes tribunals, which the United States has strongly and directly supported, also refused to immunize political or military leaders of whatever rank.

Consider the views of the victims, who recognize the hypocrisy of the United States immunizing its top leader, but not theirs, for atrocity crimes. My point is not about Mr. Trump per se but about a possible future scenario under any sitting president. It seems that the OLC and Mr. Mueller have resisted the very theory of prosecuting our government’s highest leader in a federal court while in office for the worst possible assaults on humankind when timing is everything and countless lives could be saved.

Mr. Mueller, like the OLC lawyers, ignored the worst-case scenario. … What the United States cannot afford is to undermine the global rule that sitting heads of state have no immunity from prosecution for atrocity crimes.

Mr. Trump will never face criminal prosecution while in office for anything described in Mr. Mueller’s report, even if he confronts the howls of impeachment proceedings. But we should not get comfortable with the idea—misleadingly inferred by Mr. Mueller and the OLC of decades past—that an incumbent president can avoid immediate criminal accountability for masterminding atrocity crimes. This is an important issue of constitutional interpretation.

Perhaps the OLC even today would argue that a president can proceed with atrocity crimes immune from legal accountability as multitudes perish. Mr. Mueller confirms that federal crimes could be investigated but just not prosecuted against a sitting president. If one followed Mr. Mueller’s rationale in an atrocity crime situation, then this would ignore the imperative of atrocity prevention, the consequences of delaying justice for the ever-increasing number of victims, and the humanistic values that underpin our republic’s founding documents, the Declaration of Independence and U.S. Constitution.

Did the Founders foresee the unimaginable killing machines of the modern era? Did they go as far as to envisage a president immune in waging the worst of crimes and shielded by political cowardice? I doubt it, although America’s dependence on slavery found cover in the original Constitution and the U.S. government was born out of devastation to native populations.

One small but crucial step should be taken for many sound reasons, including to correct the record created by the Mueller report and the OLC. When Congress returns from its summer break, they should take up a crimes against humanity bill (first introduced by Senator Richard Durbin (D-Il.) in 2010) to inject into the federal criminal code the many crimes (such as extermination, sexual violence, ethnic cleansing, enslavement) that are planned assaults on civilian populations and merit aggressive prosecution just like genocide and war crimes. Scores of other nations have legislated crimes against humanity and the U.N. International Law Commission is making progress on an international convention on crimes against humanity.

As momentum builds for impeachment proceedings, many in Congress will point to the examples of Mr. Trump’s obstruction of justice described in Volume II of the Mueller Report. There is sure to be discussion of the OLC opinions. Arguments both for and even against impeachment should state, for the record, that even though the OLC and Mr. Mueller do not believe that evidence about obstructing justice can be used to criminally prosecute Mr. Trump while he remains in office, there is nothing in the OLC opinions, or in Mr. Mueller’s report, that convincingly would deny contemporaneous criminal prosecution for atrocity crimes by a sitting president. Put another way, when scrutinizing the OLC opinions, it will serve the country well for legislators and other thought leaders to observe that the OLC opinions are at least demonstrably wrong when it comes to hypothetical scenarios such as the commission of atrocity crimes.

That point of view needs to be reiterated by lawmakers now so that a more fulsome interpretation of constitutional law can be delivered in the future by government lawyers, judges, and any special counsel appointed to investigate, if fate dictates, whether a sitting president ordered or orchestrated atrocity crimes and to hold him or her accountable for such diabolical presumption of power. It’s also important to enshrine this proper understanding of the law in support of the United States’ insistence in international affairs that other nations’ leaders are not immune from prosecution when they commit the most heinous acts imaginable.