The United States is currently waging an illegal war against ISIS in Iraq and Syria, an Army officer said in a lawsuit Wednesday that carries major constitutional implications for the Obama administration’s campaign against the militant group.

Nathan Smith, a U.S. Army captain and intelligence officer currently stationed in Kuwait, filed the lawsuit in a federal district court in Washington, D.C. In it, he argues President Obama violated the War Powers Resolution by continuing the conflict without congressional support within the law’s timeframe.

“The President did not get Congress’s approval for his war against ISIS in Iraq or Syria within the sixty days, but he also did not terminate the war,” the lawsuit states. “The war is therefore illegal.”

Smith’s lawsuit cites Little v. Barreme, a 1802 opinion by Chief Justice John Marshall that held naval officers liable for any unlawful acts they commit, even if ordered to commit them by the president. Anthony Kennedy cited the decision approvingly in his majority opinion for an executive-power case last term, as did John Roberts in his dissent.

Smith remains on active military service while the case is pending. In an affidavit filed along with the lawsuit, the soldier said he supported military action against ISIS but the potential illegality of the conflict weighed on his conscience.

“How could I honor my oath when I am fighting a war, even a good war, that the Constitution does not allow, or Congress has not approved?” he wrote.

Arguments that the war against ISIS is unlawful are not new. My colleague Garrett Epps wrote an excellent analysis of the situation last August, calling the military campaign “an ongoing violation of the Constitution, one of the most severe of the 21st century.”

But Smith’s legal maneuver to challenge is relatively novel. It sprung from an August 2015 Atlantic article by the Yale law professor Bruce Ackerman, who has studied the constitutional issues surrounding presidential uses of military force for a quarter-century. Ackerman is also advising Smith’s legal efforts.

A key obstacle to challenging an act’s constitutionality is finding a plaintiff who can lawfully bring a case before the courts. Without the proper justification for a lawsuit over an issue, judges will readily toss out the case before examining its merits. Legal standing, like a pair of bowling shoes, is always required.

In his Atlantic article last year, Ackerman argued existing case law could allow a soldier to challenge the legality of the war against ISIS. He pointed to a series of cases during the Vietnam War in which recently drafted soldiers sought injunctions against their deployment in what they described as an illegal war.

Two of the federal appeals courts that heard the Vietnam-era cases rejected the plaintiffs’ arguments on the merits. But, Ackerman pointed out in an interview, both courts agreed the draftees had standing to bring the lawsuits, a crucial precedent that makes Smith’s challenge possible.

The question for now isn’t who’s right and who’s wrong, Ackerman said, “but whether Captain Smith has the right to have an answer.”

Smith’s challenge may also be stronger on the merits than the Vietnam-era cases. Obama administration officials have defended the ISIS war’s legality by arguing the organization falls under two earlier congressional acts: the 2001 authorization for use of military force, or AUMF, against al-Qaeda, and the 2002 military authorization that sanctioned the Iraq War. But many legal scholars have long been skeptical of this expansive interpretation, and Ackerman pointed out the administration’s lack of legal clarity on the matter.

The White House, he noted, “has not released an opinion—a serious opinion—by the [Justice Department’s] Office of Legal Counsel or the White House Counsel that presents serious, sustained reasoning for its claim that these resolutions in 2001 and 2002 can somehow legitimate what would otherwise be a violation of the War Powers Resolution.”

By either not seeking or not releasing that legal opinion, Smith’s lawsuit argues the administration violated the Constitution’s take-care clause, since the War Powers Resolution requires the executive branch to release one within the same 60-day window as authorization.

Athough Smith’s case offers strong arguments, there are ways for the courts to avoid the substance of his claims. One counterpoint that might sway lower courts about the war’s legality, the New York Times mentioned, is that legislators implicitly sanctioned the war by voting for appropriations to fund it, even though Congress has not explicitly authorized the use of military force itself.

On the other hand, Congress has mulled multiple AUMF proposals to fight ISIS, most recently after the Paris attacks that killed 130 people last November. Those debates, and the implicit consideration they carry, could undercut the idea of indirect authorization through appropriations.

And with Congress essentially ground to a halt ahead of the 2016 elections, a legislative solution might not be forthcoming soon. But Ackerman hopes the lawsuit will prompt some questions to the presidential candidates about their views on the legal use of military force overseas. Most importantly, he hopes the case prevents future presidents from using the war against ISIS as a precedent for unilateral warfare beyond the War Powers Resolution’s constraints.

“We don’t have to go to the Supreme Court,” he said. “If Congress and the president, before or after the election, lives up to its constitutional responsibilities, then this lawsuit will have achieved its purpose.”