President Trump wants it known that — despite his recent decision to pull back the U.S. militarily back from previously Kurdish-held territory in Syria — he plans on “keeping the oil” in Syria and using American troops to do it.

If he follows through, he’ll set a dangerous precedent — and might commit a war crime.

Keeping Syria’s oil could well constitute pillage — theft during war — which is banned in Article 33 of the Fourth Geneva Convention and the 1907 Hague Laws and Customs of War on Land, which states, “The pillage of a town or place, even when taken by assault, is prohibited.” The prohibition has a solid grounding in the laws of war and international criminal justice, and the U.S. federal code, including as a sanction for the illegal exploitation of natural resources such as oil from war zones.

Trump indicates that a reason for seizing Syrian oil would be to “secure” it, preventing enemies from accessing it. That may sound savvy, militarily. But just as a practical matter, it’s not clear that America’s enemies, such as the Islamic State, even have the wherewithal to exploit Syrian oil. As the New Yorker’s Robin Wright argues, according to her conversations with Middle East experts, “the jihadist movement is not strong enough to retake and run Syria’s oil fields.”

Trump’s more grave rationale is his conception of oil as remuneration for U.S. military investment in the Middle East. In a speech Oct. 29, he said: “We want to keep the oil. $45 million a month? Keep the oil.” It mirrors a sentiment he expressed to ABC News in 2011 about Iraqi oil, saying, “You win the war and you take it. … You’re not stealing anything. … We’re taking back $1.5 trillion to reimburse ourselves.” That argument goes well beyond the notion of securing the oil — it suggests trying to profit from it — and therefore risks triggering responsibility for pillage. Contrary to Trump’s characterization, pillage is a form of stealing.

None of this is a new line of thinking for Trump: As a private citizen in 2011, in an interview with the Wall Street Journal, commenting on U.S. military involvement in Libya, he said: “I’m only interested in Libya if we take the oil. If we don’t take the oil, I’m not interested.” Regarding Iraq, he said: “I always heard that when we went into Iraq, we went in for the oil. I said, ‘Ah, that sounds smart.’ ” Indeed, he sounded disappointed during his televised announcement last week of the killing of Islamic State leader Abu Bakr al-Baghdadi, when he returned to the subject of oil and lamented: “I always used to say … ‘If they’re going into Iraq, keep the oil.’ They never did. They never did.”

In the case of the United States, specifically, acting on such a policy could put the U.S. military in the position of acting as war profiteers in foreign war zones. In the general case, taking Trump’s stated approach invites the perpetuation of resource wars that create incentives for violence on the part of belligerents the world over, at a tremendous human cost. It’s one of the reasons that, historically, prohibitions on pillage have been codified in the rules of warfare and why a number of courts have applied the prohibition to the illegal exploitation of natural resources during war.

President Abraham Lincoln commissioned Francis Lieber to draft a code of laws applicable to the Civil War, officially called the “Instructions for the Government of Armies of The United States in the Field,” also known as the Lieber Code. It proscribed, among other things, “all pillage.” After World War II, the International Military Tribunal in Nuremberg, Germany, convicted Walther Funk, the Nazi Reich economic minister and head of the Continental Oil Co., of pillaging oil seized from locations throughout occupied Europe. Likewise, the U.S. military tribunal at Nuremberg found that Paul Pleiger, a company manager installed by the German occupying army, had overseen the pillage of coal and iron ore from mines in countries occupied during the war. An appeals court judge in Singapore found that Japan’s seizure of oil stocks from the Netherlands East Indies during the war constituted “economic plunder.”

The prohibition of pillage has contemporary salience as well, giving rise to potential liabilities that are broadly diffused across political, military and corporate actors: In 2015, Belgian authorities arrested a local businessman on charges of having pillaged diamonds from Sierra Leone. In 2013, Swiss authorities opened a formal investigation into a major gold refinery on allegations of having pillaged gold from Congo. In the coming weeks, the International Court of Justice will adjudicate damages against the government of Uganda, having found that its military pillaged Congo’s natural resources in the course of a broader conflict that former secretary of state Madeleine Albright once dubbed Africa’s “first world war.”

There was a time when the prevailing view of war followed the maxim “to the victor go the spoils.” But the international community long ago came around to prohibition of pillage as necessary to ensure discipline within military ranks, to limit undue interference with property rights during war and because it created incentives for armed aggression in the first place. As a U.S. tribunal at Nuremberg articulated in 1948, “Just as the inhabitants of the occupied territory must not be forced to help the enemy in waging the war against their own country or their own country’s allies, so must the economic assets of the occupied territory not be used in such a manner.”

The United States went to war in Iraq in 1990-1991 in part to confront this problem, when Iraq invaded Kuwait seeking to control its oil. For the United States to profit from its control over Syria’s oil now would not only yoke those who help carry out this policy with the risk of violating international and federal law; it also would set a lamentable example that other nations and armed groups may follow, making the world more violent and less secure.

If Trump does seize Syria’s oil, the practice seems destined to meet with serious opposition from senior U.S. service leaders, administrators and politicians. Brett McGurk, Trump’s former envoy to the coalition tasked with defeating the Islamic State, remarked, “Oil, like it or not, is owned by the Syrian state.” Retired Gen. Barry McCaffrey bluntly summed up the stakes, tweeting:

After Trump sounded his take-the-oil refrain during a 2016 presidential candidate forum, his opponent, former secretary of state Hillary Clinton, said the United States “does not invade other countries to plunder and pillage.” Recent events are testing that ideal and many long-standing legal precedents.