National Guard personnel deployed at an ICE facility in Broadview, Illinois, near Chicago. | Jacek Boczarski/Anadolu via Getty ImagesAbout a month ago, Trump v. Illinois looked like it was going to be one of the most consequential Supreme Court decisions of the entire Trump era. The case concerns President Donald Trump’s attempt to deploy National Guard troops to an immigration facility in Broadview, Illinois, in order to suppress a small protest that has ranged in size from a few dozen people to about 200.After two lower courts ruled that Trump’s use of the National Guard in this way is illegal, Trump asked the Supreme Court — which has a Republican majority that frequently rules in his favor — to greenlight his use of troops in mid-October. Yet, in a break from their ordinary practice, the Court signaled in an October 29 order that they are skeptical of many of Trump’s legal arguments. And they asked for additional briefing on a question that neither party raised to the justices. The first round of those new briefs were filed on November 10, and the Court is likely to rule on the case after briefing completes on Monday, November 17.The upshot is that Trump’s attempt to send troops to Broadview remains blocked by the lower court orders, at least for now. The Court’s October 29 order, moreover, suggests that the justices may look to delay a showdown on whether and when Trump can use the military against Americans on US soil until a future case.Key TakeawaysTrump wants the Supreme Court to give him sweeping authority to use the National Guard against political protesters on US soil.The Supreme Court, however, signaled that it may resolve this case in a surprising way — suggesting that Trump must first use regular military forces before relying on the National Guard.There are legal restrictions on Trump’s authority to use the regular military against Americans, but the impact of those restrictions will need to be determined in a future case.For now, in other words, the stakes of the Illinois case may be somewhat diminished. But, even if the Court does rule against Trump, that will likely only punt the question of when Trump is allowed to deploy troops against Americans until a future date.So what are the legal issues in Illinois?Beginning in September, a small group of people have protested the Trump administration’s immigration policies outside of a detention facility in Broadview. According to Judge April Perry, a federal district judge who ruled against Trump’s use of troops, “the typical number of protestors is fewer than fifty,” and “the crowd has never exceeded 200.”Some members of this small group of protesters have also been charged with crimes. Some allegedly vandalized federal law enforcement vehicles, and others have been arrested for more serious crimes such as aggravated battery.When Illinois first reached the Supreme Court, the case seemed to turn on whether federal law permits using National Guard members against such a minor protest. Ordinarily, the National Guard is under the command and control of state governments. But Trump relied on a federal law that permits the president to take command of the Guard if there is “a rebellion or danger of a rebellion against the authority of the Government of the United States” or if “the President is unable with the regular forces to execute the laws of the United States.”In their initial brief to the justices, Trump’s lawyers made some very aggressive arguments. Among other things, they claimed that the decision to take control of National Guard members “is committed exclusively to the president” and cannot be reviewed by federal courts. They also claimed that the small group of protesters constitutes a “rebellion” that justifies military force.But, in their October 29 order, the justices dodged these questions and instead asked for briefing on an unrelated question: “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 relevant federal statute.The fact that the Court did not immediately rule for Trump on his more aggressive arguments suggests that a majority of the justices are skeptical of them. There would be no need, for example, to parse the meaning of the words “regular forces” if Trump’s decision to deploy troops against protesters cannot be reviewed by the courts. It is likely, moreover, that the justices are sympathetic to a reading of these two words that would require them to rule against Trump in the Illinois case. But a ruling against him on these narrow grounds would merely delay the question of whether and when Trump can use the military against Americans on US soil.What are the “regular forces”?Recall that federal law permits the president to use the National Guard to quell domestic unrest if “the President is unable with the regular forces to execute the laws of the United States.” Prior to the Supreme Court’s October 29 order, both the Trump administration and the lower courts read the phrase “regular forces” to refer to civilian law enforcement officials. Thus, as Trump’s lawyers presented the case to the justices, Illinois turned on whether Trump was unable to enforce US law in Broadview without relying on the military at all.But an amicus brief filed by Georgetown law professor Marty Lederman argues that the words “regular forces” refers, not to civilian law enforcement, but to “the standing military forces of the Armed Services, within the Department of Defense.” Thus, under Lederman’s reading, Trump may only deploy the National Guard to enforce domestic law if he has first demonstrated that he is unable to enforce the law using the actual Army or Marines.And it appears that Lederman’s argument is sufficiently persuasive that it convinced a majority of the justices to seek additional briefing from Trump and from the Illinois plaintiffs on whether Lederman is correct.To understand Lederman’s argument, it’s helpful to understand some of the history of the National Guard. Many of the United States’s founders feared a permanent, national, standing army; as the Supreme Court summarized this fear in a 1990 opinion, “there was a widespread fear that a national standing Army posed an intolerable threat to individual liberty and to the sovereignty of the separate States.”Instead, the framers imagined that states would maintain a militia that would generally operate under the state’s control. But the Constitution also permitted Congress to call this militia into federal service “to execute the Laws of the Union, suppress Insurrections and repel Invasions.”The framers’ original vision of a nation with no permanent federal army obviously no longer describes America’s relationship with the military. The United States has the most powerful standing military forces in the world. By the dawn of the 20th century, the state militias had, in the Supreme Court’s words, “proved to be a decidedly unreliable fighting force.” President Theodore Roosevelt warned that the laws governing these militias were “obsolete and worthless.”In 1901, Roosevelt asked Congress to enact new legislation that would replace the atrophied militias with what became the modern-day National Guard. Notably, Roosevelt’s message to Congress proposed that the armaments and other resources provided to this new militia should “be made identical with those provided for the regular forces.”Congress responded with the regrettably named Dick Act of 1903 (the law is named after Rep. Charles Dick, who chaired an important congressional committee), which established “the organized militia, to be known as the National Guard of the State, Territory, or District of Columbia.” As originally enacted, the Dick Act permitted the president to use the National Guard when he is “unable, with the other forces at his command, to execute the laws of the Union in any part thereof.” But Congress amended the law in 1908 to something more similar to the modern version. The 1908 version provided that the president may use the National Guard when he “is unable with the regular forces at his command to execute the laws of the Union in any part thereof.”According to the Senate Committee on Military Affairs, this change was made to ensure that the National Guard would be the “second line of defense” after the use of other military forces. (Congress removed the words “at his command” in 1956 because it deemed them to be “surplusage.”)In any event, much of Lederman’s brief is a long list of historical examples showing that Congress used the words “regular forces” to distinguish a standing federal military from state militias. An 1806 law, for example, provides that militiamen are subject to the same rules of military justice as federal troops when “acting in conjunction with the regular forces of the United States.” A broader 1814 law concerned court martials of militiamen serving under the United States “whether acting in conjunction with the regular forces or otherwise.” The Supreme Court also used the term “regular forces” in this way around the time that the Dick Act was enacted and first amended. In McClaughry v. Deming (1902), for example, the Court referred to the “substantial difference between the regular forces and the militia.”There’s more. But suffice to say, a majority of the justices appear to have found his evidence persuasive enough to set aside the Trump administration’s arguments and instead demand additional briefing on the meaning of the term “regular forces.”What does it mean if Lederman is right about the words “regular forces?”Unless you are a fan of military forces in US streets, the implications of Lederman’s argument may seem alarming. If Trump isn’t allowed to use the National Guard without first attempting to use the regular Army or the Marines, then he is likely to deploy the regular Army or the Marines against Americans protesting his policies.But there are other laws that govern the use of regular military forces within the United States. One is the Posse Comitatus Act, which prohibits the use of those forces “to execute the laws” except “under circumstances expressly authorized by the Constitution or Act of Congress.” Another law, commonly known as the Insurrection Act, permits the president to use the military to “suppress, in a State, any insurrection, domestic violence, unlawful combination, or conspiracy” — but only in limited circumstances laid out in that statute.The Justice Department has long interpreted these circumstances very narrowly. In a 1964 memorandum signed by then-Deputy Attorney General Nicholas Katzenbach, for example, the DOJ concluded that the Insurrection Act may only be used if “state authorities are either directly involved, by acting or failing to act, in denials of federal rights of a dimension requiring federal military action, or are so helpless in the face of private violence that the private activity has taken on the character of state action.”Barring a court order permitting the use of troops, moreover, Katzenbach concluded that military force is only allowed under the Insurrection Act when “those engaging in violence are either acting with the approval of state authorities or have, like the Klan in the 1870s, taken over effective control of the area involved.”Needless to say, the small group of protesters in Broadview hasn’t come close to seizing control of that region in the same way that the Ku Klux Klan dominated many parts of the South in the post-Civil War era.Still, the fact that the Justice Department has long interpreted the Insurrection Act narrowly does not mean that either the Trump administration or this Supreme Court — which is ordinarily so sycophantic toward Trump that it literally held that he is allowed to use the powers of the presidency to commit crimes — will read the same law as narrowly.But a legal showdown over the meaning of the Insurrection Act is not currently before the Court. If the justices accept Lederman’s interpretation of the amended Dick Act, then that will mean Trump will lose his bid to send National Guard members to Broadview. It will then be up to Trump to decide if he wants to set off a new legal fight by claiming authority under the Insurrection Act, and deploying regular forces to American cities.