Trump has a long history of calling for the U.S. to "take the oil" in the Middle East, in Iraq and Syria in particular. But any oil in both countries belongs to their governments, and according to U.S. law and treaties it has ratified, seizing it would be pillaging, a technical term for theft during wartime that is illegal under U.S. and international law.
"We're keeping the oil," Trump said Monday to a conference of police chiefs in Chicago. "I've always said that -- keep the oil. We want to keep the oil, $45 million a month. Keep the oil. We've secured the oil."
On Sunday, when detailing the U.S. special forces raid against ISIS leader Abu Bakr al-Baghdadi, Trump said U.S. troops would remain in Syria to secure "massive" oil reserves and even put up "a hell of a fight" against any force that tried to take them.
"We should be able to take some also, and what I intend to do, perhaps, is make a deal with an ExxonMobil or one of our great companies to go in there and do it properly," he added.
Defense Secretary Mark Esper confirmed Monday that U.S. troops will remain in the eastern Syrian province of Deir ez-Zor "to secure the oil fields" against ISIS. But the senior State Department official said the administration was "just beginning to look at the specifics of this," and downplayed a U.S. role in seizing any oil.
Instead, the official implied that the Syrian Democratic Forces, the majority-Kurdish forces that fought with the U.S. against ISIS, would continue to extract and profit off the oil produced in the area.
"The SDF has been operating these oil fields for some time and has their own arrangements with various actors on who it gets sold to and such, and we haven't been too involved in that," the official said.
Most of that oil is sold to the regime of Bashar al-Assad, the strongman who has waged a war against his own people and is opposed by the U.S.
If U.S. troops or companies were to take any oil without Assad's government's permission, it would be considered pillaging, according to legal experts, because the land and its resources belong to Assad, who despite the eight-year civil war remains the country's head of state.
Pillaging is illegal under international law, explicitly prohibited by the Fourth Geneva Convention, which the U.S. ratified as a treaty in 1955. The U.S. War Crimes Act of 1996 also made it punishable under U.S. law to commit a "grave breach" of any of the Geneva conventions "whether inside or outside the United States."
These codifications were built on many previous legal prohibitions and military practices, from the charter of the Nuremberg trials that prosecuted the Nazis after World War II, to the Hague Convention of 1907 which was first proposed by President Theodore Roosevelt, all the way back to the 1863 Lieber Code. Commissioned and signed by President Abraham Lincoln, it governed the conduct of the Union Army in the field during the American Civil War and prohibited "all pillage or sacking, even after taking a place by main force," punishable by death.
Although the U.S. is not a signatory to the Rome Statute that established the International Criminal Court in 1998, 122 other countries are and could extradite American officials if required to by the court -- something the Trump administration has vociferously battled in a possible ICC investigation into war crimes by the U.S., the Taliban and other forces in Afghanistan.
Still, the senior State Department official said securing oil fields is simply part of U.S. forces' fight against ISIS, not any U.S. effort to extract oil.
"It's very important to keep that out of the hands of ISIS, given ISIS's history of fueling and funding its caliphate with those oil fields," they told reporters Monday, adding, "The revenues generated by that allow the SDF to operate as a security and governance entity in the northeast, which thus contributes to our platform of D(efeating) ISIS there."
The administration has said those U.S. forces in northeastern Syria are there to combat the terror group ISIS -- arguing that means their deployment is within the authorization for the use of military force, or AUMF, that Congress passed in 2001. That's debatable, according to many Republicans and Democrats in Congress, as that law was passed specifically to counter al-Qaeda and those that aided it with the Sept. 11 attacks, but saying U.S. forces are now there to secure oil fields has raised greater alarm among lawmakers about the legality of their deployment.
In particular, Trump threatened to use military force to defend U.S. control of the oil fields, saying, "Either we'll negotiate a deal with whoever is claiming it, if we think it's fair, or we will militarily stop them very quickly."
The Pentagon confirmed that U.S. forces would "respond with overwhelming military force against any group who threatens the safety of our forces there," according to Esper, even if it were Assad or his backers Russia and Iran. But that kind of fight over oil fields would not be permitted under the AUMF, setting up at the very least a legal battle with Congress.
"Congress never authorized the troops in Syria in the first place, let alone troops to protect oil fields. This is unconstitutional," tweeted Rep. Ro Khanna, D-Calif.
Beyond the legality, many critics have also said Trump's initial decision to withdraw U.S. forces and abandon the SDF that lost 11,000 fighters as the de facto U.S. foot soldiers against ISIS, but now keep some hundreds of troops to guard oil further undermines America's standing.
"We'll betray an ally, but we'll go back in to protect the oil? That sickens me, frankly, and I think as you can see from so many of the troops that have had to pull out and abandon their allies, they're sickened by the president's decision as well," Rep. Adam Schiff, D-Calif., told ABC's "This Week" on Sunday.
ABC News' Terry Moran and Elizabeth McLaughlin contributed to this report.