Late Tuesday, the U.S. Supreme Court delivered a significant setback to the Trump administration in a high-profile case concerning the president’s authority to deploy National Guard troops to support Immigration and Customs Enforcement (ICE) agents involved in large-scale deportations.

The justices, in a majority decision, upheld a temporary restraining order issued earlier in October by U.S. District Judge April M. Perry, an appointee of President Joe Biden. This order prevented the federalization and deployment of the National Guard in Illinois.

In response, the Trump administration swiftly appealed to the U.S. Court of Appeals for the 7th Circuit, seeking emergency relief. They requested both an immediate administrative stay and a more comprehensive stay pending the outcome of the case.

The appeals court’s decisions were mixed. Initially, a panel granted an administrative stay on the federalization directive but allowed Judge Perry’s ban on troop deployment to remain intact. When the court reviewed the case’s merits, they maintained their original ruling.

While the Trump administration has enjoyed a favorable track record with the Supreme Court’s emergency docket, often referred to as the “shadow docket,” this particular case marked a rare defeat for the government.

The opinion – which appeared to be 6-3; the exact vote was not published – hinges on the definition of “regular forces” in federal law.

At the district court level, there was some contention over a subsection in 10 U.S.C. §12406, which empowers the president to federalize members of the National Guard if he is “unable with the regular forces to execute the laws of the United States.”

Notably, in their initial briefs before the high court, neither party felt compelled to define the term “regular forces.”

Justice Amy Coney Barrett, however, noticed that the district court’s order considered a definition of “regular forces” that did not favor the government. Barrett then directed the parties to provide supplemental briefing on whether “the term ‘regular forces’ refers to the regular forces of the United States military, and, if so, how that interpretation affects the operation” of the statute in question.

In supplemental letter briefs, the parties disputed the definition.

The government argued the term “does not refer to the standing military” but “refers instead to the civilian forces that regularly ‘execute the laws’ at issue but are ‘unable’ to do so in present circumstances.” Meanwhile, Illinois argued the term “refers to the full-time, professional military.”

A majority of the high court sided with the Land of Lincoln.

The unsigned opinion, typical of shadow docket rulings, says the term “likely refers to the regular forces of the United States military.”

“This interpretation means that to call the Guard into active federal service under §12406(3), the President must be ‘unable’ with the regular military ‘to execute the laws of the United States,’” the opinion reads. “Because the statute requires an assessment of the military’s ability to execute the laws, it likely applies only where the military could legally execute the laws. Such circumstances are exceptional.”

The majority goes on to suggest the National Guard issue here might prove especially difficult for the president to resolve.

“Under the Posse Comitatus Act, the military is prohibited from ‘execut[ing] the laws’ ‘except in cases and under circumstances expressly authorized by the Constitution or Act of Congress,’” the opinion goes on. “So before the President can federalize the Guard under §12406(3), he likely must have statutory or constitutional authority to execute the laws with the regular military and must be ‘unable’ with those forces to perform that function.”

The opinion goes on like this, at length:

[T]he Government has failed to identify a source of authority that would allow the military to execute the laws in Illinois. The President has not invoked a statute that provides an exception to the Posse Comitatus Act. Instead, he relies on inherent constitutional authority that, according to the Government, allows him to use the military to protect federal personnel and property. But the Government also claims—consistent with the longstanding view of the Executive Branch—that performing such protective functions does not constitute “execut[ing] the laws” within the meaning of the Posse Comitatus Act. If that is correct, it is hard to see how performing those functions could constitute “execut[ing] the laws” under §12406(3).

In other words, the president needs constitutional or statutory authority to use the regular military for domestic matters; the president must then be unable to use this theoretical version of the military for police work. Only then can National Guard troops be federalized.

However, the court also notes that the government has not cited any source of authority to call up the military under such circumstances – and suggests that any such citations might be difficult to find. This is because the government has already exhausted one source of authority to make a different argument altogether.

The court’s atypical ruling here exceeds the appeals court’s ruling, which did allow the basic federalization of National Guard troops. This would seemingly frustrate similar federalization and deployment plans in other states beyond Illinois, though using shadow docket rulings as precedent is not something to take for granted.

In dissent, Justice Samuel Alito – joined by Justice Clarence Thomas – sharply disagreed with how his colleagues handled the case.

“In this case, the Court has unnecessarily and unwisely departed from standard practice,” the dissent begins. “It raised an argument that respondents waived below, and it now rules in respondents’ favor on that ground.”

Alito says the high court “in effect adds new language to the text Congress enacted” so that now the relevant statute includes a “restriction” on presidential power over the National Guard.

To hear Alito tell it, the parties early on agreed “regular forces” refers to “civilian federal law enforcement officers” such as the FBI, DEA, and so on, “not military service members.” And, when confronted with the possibility of disputing the term, the issue “was either waived or forfeited three times over.”

“In this case, the Court has no good reason to stray beyond the issues that the parties chose to present, and based on those arguments, the Court should grant the application,” Alito goes on. “There is no basis for rejecting the President’s determination that he was unable to execute the federal immigration laws using the civilian law enforcement resources at his command.”

Alito then spends several pages disagreeing with the lower court’s analysis, staking out a case for strong presidential power, and stressing the need for ICE agents to receive federal support.

“Local police departments have provided insufficient assistance,” Alito rages in dissent. “[T]he protection of federal officers from potentially lethal attacks should not be thwarted.”

Justice Neil Gorsuch penned a separate dissent saying he would rule in the government’s favor based on the earlier, agreed-upon definition of “regular forces,” but “would hazard no opinion beyond that.”