We have known for a while that Trump has been itching to use military force in civilian settings, and he’s finally found an opportunity in the ICE protests currently taking place in Los Angeles. On Saturday, President Trump issued a presidential memorandum, federalizing the National Guard, putting Defense Secretary Pete Hegseth in charge, and bypassing California Governor Gavin Newsom in the process. It’s a fluid situation, and Governor Newsom has since sued the President for overstepping his statutory authority. Given the various authorities in play, including ones that have not yet been used, it’s worth unpacking what is actually happening, where it could go, and where it can’t.

Let’s start with the basics. As a general matter, under the Posse Comitatus Act of 1878, the federal military is not allowed to be used for domestic law enforcement unless a specific exception is made through an act of Congress, or otherwise authorized under the Constitution. Posse Comitatus applies to federal, but not state, troops – so it ordinarily would not apply to the National Guard when called into service by a governor of a state. Posse Comitatus was originally passed after Reconstruction to put an end to the military’s role of enforcing civil law, as it had immediately after the Civil War. However, the prohibition reflects the more general aversion to the potential for abuse of a standing army codified in parts of the Constitution (like the Standing Army Clause, which puts a two-year limit on appropriations for the Army, or the Third Amendment, which prohibits the quartering of troops), which stemmed from the Framers’ own experiences with the British army as colonists. 

The worry about executive control of a standing army is also reflected in how the Constitution addresses the role of state militias, today’s National Guard. Article I, Section 8, Clause 15 gives Congress the power “to provide for calling forth the Militia to execute the Laws of the Union, suppress Insurrections, and repel Invasions.” You’ll note that slightly weird wording there – Congress doesn’t directly call forth the militia, but “provides for” the calling forth of the militia…to someone else. As with several other provisions of the Constitution (like the Habeas Corpus Clause), this reflects a certain ambivalence among the Founders, who recognized that giving the President the direct power to call forth state militias could be problematic, but also that practically speaking, the executive would be the branch best positioned to actually identify the need for it. So here we are, and Congress has indeed passed several statutes that provide for the President to call forth the states’ militias in certain circumstances.

One of them, 10 U.S.C. Sec. 12406, is what Trump invoked in his memorandum. Section 12406 authorizes the President to federalize the National Guard in three situations: first, in case of an “invasion by a foreign nation”; second in the event of a “rebellion against the authority of the Government of the United States”; and third, if the President is “unable to execute the laws of the United States” using “regular forces.” President Trump’s memorandum appears to rest on the second justification, stating, “To the extent that protests or acts of violence directly inhibit the execution of the laws, they constitute a form of rebellion against the authority of the Government of the United States.” (Bookmark that word “rebellion.”) Here is the catch: Once a state’s national guard is “federalized,” the same limitations that apply to federal troops apply to the newly federalized Guard troops, which means they, too, are limited by the Posse Comitatus Act.

Trump’s memorandum also authorizes “the Secretary of Defense [to] employ any other members of the regular Armed Forces as necessary to augment and support the protection of Federal functions and property in any number determined appropriate in his discretion.” This part isn’t specifically authorized by Section 12406. Rather, the executive branch has claimed for over fifty years that it has “inherent authority” under Article II of the Constitution to use the military to protect federal personnel and property. (This was the authority under which Trump sent federal troops to Portland, Oregon during the George Floyd protests in his first term.) And, since in this statutory scenario the federalized National Guard troops are essentially bound by the same limitations as federal troops their function is limited to protecting federal (ICE) officers and federal property, which means they technically can’t jump in and help ICE actually carry out immigration laws. This limitation is, in fact, spelled out in Trump’s memorandum.

But there’s one hiccup: Section 12406 (the statute at issue here) states that orders to call in the National Guard under this provision “shall be issued through the governors of the States,” which clearly didn’t happen here. Newsom has cited this omission in his lawsuit. The governor’s lawsuit also argues that federalizing the California National Guard without his permission violates the Tenth Amendment by depriving the state of its sovereign rights under the Constitution. Finally, the lawsuit contests the grounds for invoking 12406 at all, declaring that there is neither an invasion by a foreign country nor a rebellion currently underway in California. (The California Attorney General’s press release observes that the only other time in modern history this statute has been invoked on its own was when President Nixon called the National Guard to deliver mail during the 1970 Postal Service Strike.)

So is this going to be settled by the courts? Maybe – unless Trump plays his, er, trump card. That would be the Insurrection Act, which predates the Posse Comitatus Act and serves as an exception to it. The Insurrection Act permits the President to use the military for federal law enforcement if “unlawful obstructions, combinations, or assemblages, or rebellion against the authority of the United States, make it impracticable to enforce the laws of the United States in any State by the ordinary course of judicial proceedings.” You might have noticed that word again, “rebellion.” (There are other grounds for invoking the Insurrection Act, but this one would be most likely in this context, especially given the language of the presidential memo.) The Insurrection Act would be an escalation in L.A. because it would allow the military – and the federalized National Guard – to go beyond their “protective” detail and affirmatively assist ICE in immigration enforcement. It would also likely moot Governor Newsom’s lawsuit because the “rebellion” grounds in the Insurrection Act don’t require a state request to send in federal (or already federalized) troops. Indeed, this was the authority President Lyndon Johnson used to federalize the National Guard to enforce civil rights laws in Alabama without Governor George Wallace’s request or consent.

So, it could get worse. One thing worth clarifying, though, is even invoking the Insurrection Act is not tantamount to “declaring martial law.” Martial law is an extreme, emergency measure that involves supplanting civilian government with one run entirely by the military. This would mean the military would be fully in charge, with soldiers replacing (not merely assisting) regular law enforcement and military courts and tribunals replacing civilian courts. There is no explicit constitutional authority for the imposition of martial law, though it has been declared at least 68 times in U.S. history (both by the federal government and by states), including in military-occupied territories during wartime (like the Civil War), or under emergency conditions (as President Roosevelt did in Hawaii after the attack on Pearl Harbor). Whatever conditions might warrant it, though, the ones in L.A. aren’t it. 

Still, the specter of armed troops in American cities is enough to make it feel like the military is in charge, and that is antithetical to a freely functioning democracy. On this note, perhaps the most alarming aspect of Trump’s memorandum is that it is not geographically limited to California – Trump is purporting to federalize all National Guards, perhaps with the intent to send troops into other cities as well, without the state’s consent. Trump’s actions are intended for spectacle and provocation. How far Trump intends to push the limits of his Commander-in-Chief authority remains to be seen, but at some point, even the military may refuse to go along.