Every once in a great while, an amicus brief filed in the U.S. Supreme Court is a gamechanger. Such amicus briefs are extraordinarily rare. Usually, amicus briefs provide an additional perspective without contradicting or diverging too sharply from the party they support. An amicus brief submitted by law professor Martin S. Lederman, on the other hand, tells the Supreme Court that it should decide a case on an independent ground that is different from those argued by the parties. If the Court agrees, its decision would upend the litigation entirely.
The case is Trump v. Illinois (25A443), which, as of this writing, is pending before the U.S. Supreme Court following an emergency application filed by the Trump administration. At issue is whether and when the President may federalize the National Guard, bringing servicemembers in state service under his command. The President has federalized and deployed National Guard troops in California, Oregon, and Illinois, among other places, over the strenuous objections of the governors of those States. Each of those States challenged the deployments in federal court, but none of them pressed the key statutory argument that Lederman made in the Supreme Court.
The statute, 10 U.S.C. § 12406, authorizes the President to federalize the National Guard and bring it under his command in cases of invasion, rebellion, and when “the President is unable with the regular forces to execute the laws of the United States.” California, Oregon, and Illinois all argued that regardless of how one defines “regular forces,” the President could not show that he was “unable” to execute the laws. Lederman argues that instead of deciding what “unable” means, the Supreme Court should decide that “regular forces” means the standing army, that is, the active-duty members of the armed services. And because no evidence shows that the President is unable to execute the law with the assistance of the military forces (and the other statutory conditions do not exist), he had no authority to federalize the National Guard.
Lederman provides support for his thesis in the statute’s history, congressional reports, and case law. And Oregon, at least, may have found it convincing. After Lederman filed his brief, on October 22, Oregon filed a brief in the Ninth Circuit noting an issue that the court had not yet addressed: “whether ‘regular forces’ means … the soldiers and officers serving in the regular armed forces, as the Seventh Circuit has recognized may be more correct.”1 California did not follow Lederman’s lead. It argued in open court that “regular forces” means “federal civilian personnel.”2
It is not clear why the States did not argue, as Lederman does, that “regular forces” means the standing military forces. There are two possibilities: either the States were unaware of Lederman’s interpretation and failed to advance it, or they disagree that his view is correct or strategically beneficial. Either one seems possible.
As for the first, the litigation issues are novel and the timeline was extremely compressed in the district court. And having not advanced Lederman’s interpretation of “regular forces” in the district court, the States may have been reluctant to raise a new case-dispositive argument in the appellate court. The meaning of “regular forces” should not matter, as they argued, because there was no exigency that left the President unable to execute the law.
It is also possible, and more likely, that the States do not agree with Lederman’s interpretation or do not find it useful or desirable. The States may prefer the deployment of the National Guard over the deployment of active-duty military force. When the President federalizes the National Guard under 10 U.S.C. § 12406, there is another law, the Posse Comitatus Act, that prohibits the Guard from performing law enforcement functions. But if the President invokes the Insurrection Act as a basis for sending the standing army into American cities, the public would lose those protections against military law enforcement. Thus, it may be better for the States to withhold Lederman’s interpretation and maintain the protections of the Posse Comitatus Act under a more limited federal deployment.
If this latter scenario is correct and the U.S. Supreme Court nevertheless takes up Lederman’s invitation to decide the case on the independent ground advanced in his amicus brief, it could mean that he blew up the States’ best laid plans, winning what might amount to a Pyrrhic victory. The U.S. Supreme Court could do exactly that, not least because Judge April M. Perry, already ruled that “regular forces” means the “standing army” in her detailed opinion. If it did, that ruling would effectively invalidate the deployments in California and Oregon, but on legal grounds that may not prove beneficial in the long run. As the States may have concluded, the downside of the “regular forces” argument could call for an irregular strategy to set that meritorious argument aside. The better result would be for the courts to rule as the States have requested: that under any possible interpretation of the statute, the district court did not err in finding that no evidence supports the President’s decision to federalize the National Guard.
This argument appears in Oregon’s supplemental brief submitted in the Ninth Circuit on October 22, 2025, at page 34, footnote 4, and cites Professor Lederman’s amicus brief. See Ninth Circuit Case No. 25-6268, Dkt. No. 70.1.
The Solicitor General for the State of California made this argument before a three-judge panel of the Ninth Circuit at oral argument on October 22, 2025. The video is available athttps://www.ca9.uscourts.gov/media/video/?20251022/25-3727.