A federal appeals court is asking attorneys in a prominent war-powers case to file briefs on whether the passage of time has mooted a former Army captain’s lawsuit claiming the war against the Islamic State group is illegal.

A three-judge panel heard oral arguments in the case in October, but has waited more than six months to rule, allowing time for military operations in Syria and Iraq to wind down and for the litigant, former Army Capt. Nathan Michael Smith, to leave the military.

In a Thursday order, the U.S. Court of Appeals for the D.C. Circuit instructed Smith's attorneys to file a supplemental brief by May 17. A government response is due a week later, with Smith given a final opportunity to comment by May 31.

The one-page filing says, "It is ORDERED, on the court’s own motion, that the parties file supplemental briefs addressing whether appellant Smith’s case will become moot and should be dismissed when he completes his military service obligation in May 2018."

Smith, formerly deployed as an intelligence analyst in Kuwait, left active-duty military service in June, but remains on “ready reserve” status until later this month.

Smith sued former President Barack Obama in 2016, pursuing a rarely invoked argument for standing in war-powers cases, that he has a right to sue because as a deployed soldier he was directly impacted by the allegedly illegal war. The lawsuit is now known as Smith v. Trump, reflecting the change in administrations.

Smith said he supported the fight against the jihadi group, but that his conscience bothered him, arguing he was forced to violate his oath to uphold the Constitution by participating in a war effort that was not authorized by Congress.

The Obama and Trump administrations have argued the war against ISIS is authorized by 2001 legislation authorizing military force against al Qaeda, though skeptics point to a violent rivalry since 2014 between the groups. Overlapping authority in Iraq is claimed from an unrepealed authorization passed in 2002 against former leader Saddam Hussein.

U.S. District Judge Colleen Kollar-Kotelly ruled against Smith in November 2016, finding that he lacked standing, that his claims dealt with a matter traditionally handled by the political branches of government, and that Congress had implied approval by providing funding.

At an October hearing, a Trump administration attorney argued that wars are off-limits to court review, even if they plainly are not authorized by Congress.

David Remes, an attorney for Smith, declined to comment on the new court order. Yale Law professor Bruce Ackerman, who also represents Smith, did not immediately respond to a request for comment.

Smith, who plans to enter business school this year, did not immediately respond to an email.

Although ISIS holds only small pockets of territory — after being driven back from controlling nearly a third of both Iraq and Syria — about 2,000 U.S. troops remain embedded with Kurdish militia in Syria, with additional U.S. troops in Iraq. The question of war powers in the region remains hotly contested, including after President Trump bombed Syria’s government last month after an alleged use of chemical weapons.

“My fear about exactly this sort of unilateral military action being allowed in the future was possibly a contributing factor in my lawsuit,” Smith told the Washington Examiner ahead of the April military strikes.

“For me, it’s not a partisan issue,” he said outside of the appeals court hearing last year. "Procedure matters, if we're a nation of laws.”