The undeclared U.S. war against the Islamic State group enters its third year Sunday, still lacking specific congressional authorization as it faces a potentially groundbreaking lawsuit from an active-duty Army captain who argues the military campaign is illegal.

A ruling in Nathan Michael Smith's case could come as early as this fall, as voters consider electing a vice president who is an aggressive critic of the war’s legality or making president a major-party candidate who says U.S. meddling has destabilized Iraq and Syria.

Smith, an intelligence officer based in Kuwait, says he is being forced to violate his oath to uphold the Constitution by participating in the war and hopes to overcome a standing hurdle that historically has killed war powers lawsuits.

Smith filed the lawsuit in May and the Justice Department offered a detailed counterattack in a motion to dismiss last month, questioning Smith's standing, claiming courts should not address such matters and arguing the war actually is authorized.

“To resolve this challenge, the court not only would have to pass judgment on the president’s determination that ISIL is an authorized military target, but it would have to do so in the midst of a live armed conflict, without the relevant expertise or access to the type of information necessary to render an informed decision,” government lawyers wrote, using an alternative name for the Islamic State group.

The Obama administration claims a 2001 authorization for use of military force (AUMF) passed after 9/11 allows fighting against the Islamic State group in both Iraq and Syria. The administration says a second AUMF, passed in 2002 to provide a legal basis for toppling Saddam Hussein, offers overlapping authority for military efforts inside Iraq, though the White House supports repealing the authorization.

Many legal scholars scoff at Obama’s claimed legal authority, pointing out the 2001 AUMF specifically targeted supporters or members of al-Qaida – against which the Islamic State Group has fought in Syria since 2014 – and say the 2002 authorization had a long-deceased despot in mind.

But among scholars who do believe the war is unjustified, there’s further division on whether Smith – or anyone – could win a ruling against Obama in court.

According to the War Powers Resolution of 1973, Congress must authorize military force within 60 days of the president ordering hostilities, and forces must be withdrawn within an additional 30 days if it does not come. The law was passed after former President Richard Nixon secretly expanded the Vietnam War into Cambodia.

But standing typically kneecaps challenges to the legality of wars that lack specific congressional authorization, resulting, for example, in the dismissal of a lawsuit brought by 10 members of Congress against Obama's 2011 intervention in Libya. The lawmakers claimed to have standing on the basis of their official responsibilities or their position as taxpayers.

Bruce Ackerman, the Yale Law School professor who theorized an active-duty soldier would have a rare opportunity to challenge the war by meeting the Constitution's standing requirements, says he expects U.S. District Judge Colleen Kollar-Kotelly to issue a ruling about the time of the November election.

Ackerman says a best-case scenario is a declaratory judgment from Kollar-Kotelly that the war is unauthorized, providing for a reset on the War Powers Resolution clock to allow Congress 60 days to pass a new AUMF.

Obama himself almost certainly would face no consequences if the war is found illegal.

Army Captain Nathan Michael Smith filed a lawsuit in May, claiming the U.S. war against the Islamic State group lacks legal authorization. U.S. Army

“This is a serious person making a serious decision that is legally wrong,” Ackerman says. "It’s not an impeachable offense, it's not a high crime or misdemeanor. It’s just a terrible mistake [and if unchecked] every future president will point to Obama's decisions to violate the War Powers Resolution as a precedent for their own unilateral war making.”

Among the fiercest critics of the Obama administration’s claimed authority to wage war without a specific legislative authorization is Sen. Tim Kaine of Virginia, who recently became the Democratic vice presidential nominee.

Though Obama also articulated a vision of limits on presidential war-making while a senator, Kaine has pointedly and repeatedly pressed the matter.

“ISIL is not al-Qaida, nor is it an associated force,” Kaine said in a June 2014 speech on the Senate floor, before Obama ordered U.S. intervention. “While it forged a temporary alliance with al-Qaida in 2004, three years after 9/11, it is now an avowed enemy of al-Qaida, and it's viciously battling al-Qaida in Syria as we speak. It would be a wholly unprecedented stretch to suggest that the 2001 AUMF would justify action against ISIL in Iraq.”

Almost two months after Obama launched airstrikes against the Islamic State group in Iraq, Kaine delivered a major speech insisting a new authorization was needed for the effort to be legal.

“To call ISIL an associated force with al-Qaida, when they have separated from al-Qaida, when, in parts of Syria, they are battling al-Qaida, again is such a torturous read of the language that I don’t believe either a clear reading of the original language or even the broadening gloss put on it by the administration would allow ISIL to be fairly incorporated into the 2001 AUMF,” he said. “The administration has claimed that this ’02 authorization somehow justifies this action against ISIL, at least in Iraq; again, I think that argument is specious. The purpose of that ’02 authorization was not to engage in open-ended war in the ZIP codes that happen to be Iraq without limitation in terms of time. It was directed at the toppling of a particular regime.”

When Obama ultimately did propose a draft AUMF for the ongoing war effort in the Middle East, it did not propose repeal of the 2001 authorization, and Kaine criticized the offering’s “breadth and vagueness.” It's become one of his distinguishing policy positions, which he's addressed repeatedly within Congress and in media interviews, and one of the few areas in which the staunch Obama ally has publicly disagreed with the president.

The Justice Department's motion to dismiss offers many outs to a judge inclined to sidestep the core questions raised by Smith’s lawsuit, arguing that the case addresses a non-justiciable political question and asserting Congress has essentially approved the war as evidenced either by lack of conflict or approved military spending. Harvard Law School Professor Jack Goldsmith, a skeptic of Obama's reliance on dated AUMFs, said after Smith filed his lawsuit that such points could allow a government win.

The Justice Department also fired back that Smith is not suffering an injury that would establish standing.

“A service member may face the dilemma of violating his oath if confronted with an order to carry out a patently illegal act during war. But the same dilemma does not arise from a mere disagreement as to whether Congress has lawfully authorized a military operation,” the motion to dismiss argues. “Indeed, to find a cognizable injury based on a service member’s oath in this setting would turn the military’s system of order and obedience on its head.”

Ackerman says, however, that Smith takes his oath very seriously and would be forced to make a tough personal decision if the case is tossed. Like Kaine, Smith supports military efforts against the Islamic State group and says Congress should pass legislation to make the war lawful.

“Captain Smith is in favor of this war on the merits,” Ackerman says. “This is the kind of war he signed up for. The question is whether the president is governed by the rule of law.”

The Yale Law School professor says Smith continues to be treated well by his colleagues in Kuwait.

Smith’s other lawyer, David Remes, well-known for his legal work on behalf of Guantanamo Bay detainees, told U.S. News earlier this year he was told by Smith that fellow soldiers' reactions fell into three categories: “overtly supportive, discreetly supportive and the third group is an X-factor, they don't say one way or the other.”

It's unclear if a soldier serving on the front lines advising Iraqi troops or militia, Syrian rebels or flying air support missions would have a stronger legal argument. Other prospective plaintiffs contacted Ackerman, he says, after he floated an outline of the lawsuit last summer and then published a more detailed legal analysis in The Atlantic.

Ackerman says it's too soon to know when precisely Kollar-Kotelly, who is receiving another round of paperwork due in September, will issue a ruling in the lawsuit.

If Donald Trump becomes president, it's unclear how he would approach the lack of specific congressional authorization. He repeatedly criticizes Obama and Democratic nominee Hillary Clinton for intervening in Syria and Iraq, while faulting them for being insufficiently tough on the Islamic State group.

Ackerman says, however, he believes a Vice President Kaine might accomplish the same thing as a legal triumph.