A shallow river
In January 49 BC, Julius Caesar stood with the Thirteenth Legion on the bank of a small river called the Rubicon.
The river lies near present-day Rimini, in northeastern Italy. It is neither wide nor deep. What made it matter was not the water but the law: the Rubicon marked the boundary between the province of Cisalpine Gaul and Italy proper. Under Roman rules, a provincial governor’s military command was valid only inside his province. Leading an army across that line into Italy was treason against the Republic.
So what stood in Caesar’s way was never the water. It was an agreement. Romans shared the belief that this line could not be crossed with an army. The agreement had no walls and no garrison; its only guarantee was that generals were willing to respect it, and that soldiers would refuse to follow anyone who did not.
Why did Caesar cross? Later writers often tell the scene as an ambitious man’s gamble. But the ancient sources point to a more practical motive as well: fear.
Caesar had fought in Gaul for nearly ten years, and his enemies in Rome had been waiting almost as long. The Senate demanded that he give up his command and return to Rome as a private citizen. But a private citizen had no immunity from prosecution. According to Suetonius, Cato the Younger swore more than once that he would haul Caesar into court the moment he dismissed his army. A year and a half later, at Pharsalus, looking out over the fallen Pompeian soldiers, Caesar said:
This is what they wanted. After achievements so great, I, Gaius Caesar, would have been condemned had I not sought help from the army.
It was, of course, self-justification. But it lays bare the most dangerous logic of a republic on the verge of collapse: when losing power means losing your freedom, those who hold power will no longer give it up willingly.
More than two thousand years later, I often think about this river. Not because I believe any American president is Caesar—historical analogy is not evidence, and America is not Rome. But because it raises a question that still holds: a border with no walls—what keeps it in place?
Put another way: where is America’s Rubicon? And is it still there?
Before the crossing, the river had already run shallow
Caesar was not the first to lead an army into Rome. Long before him, the river had been tested many times.
From the time of Marius onward, Roman legions were increasingly recruited from the landless poor. Whether a veteran received a plot of land to retire on often depended on whether his general could win it for him in Rome. The soldier’s livelihood became bound to the general’s political fortunes, and his loyalty slowly shifted from the Republic to the general.
In 88 BC, after his political enemies stripped him of command in the war in the East, Sulla marched on Rome. It was the first time a Roman army had attacked its own capital. According to Appian, nearly all of Sulla’s officers abandoned him; only one quaestor stayed. They knew the line could not be crossed. But the soldiers followed Sulla.
Over the next forty years, Marius and Cinna retook Rome by force; Sulla fought his way back a second time and published his proscription lists, putting bounties on his enemies’ heads and seizing their property. The price of political defeat went from losing office to losing one’s life and estate. By the time Caesar stood on the bank of the Rubicon, Romans had seen the line crossed too many times.
This time, the most important figure to leave Caesar was Labienus, his ablest lieutenant in Gaul, who went over to Pompey. The legions stayed. Caesar told his soldiers that the Senate had trampled on the rights of the tribunes of the plebs, and that he was crossing to defend them—in other words, the man crossing the line claimed to be protecting the Republic.
In hindsight, Caesar’s crossing was less a sudden coup than a confirmation: confirmation that the river had grown too shallow to stop anyone. Sulla’s officers could still refuse; Caesar’s soldiers no longer hesitated. Every crossing that went unpunished made the next one easier; every political struggle settled by trials and violence made the next person in power more afraid to let go.
A republic rarely loses its borders in a single flood. More often, the river runs shallow a little at a time.
Americans wrote the river into an oath
America’s founders had read their Roman history. What they feared most was exactly this: a standing army becoming one man’s private force. So the Constitution made the president commander in chief, but gave Congress the power to raise armies, fund them and declare war. The military was to obey elected civilians, but not any one particular person.
The line is written most clearly in an oath. When an American military officer is commissioned, the oath has a single object of loyalty: to “support and defend the Constitution of the United States against all enemies, foreign and domestic” and to “bear true faith and allegiance to the same.” The president does not appear anywhere in it. Neither does anyone’s name.
The enlistment oath adds a promise to obey orders, but its order and its qualifications are deliberate: first, defend the Constitution; only then obey the orders of the president and of superior officers—and that obedience must be “according to regulations and the Uniform Code of Military Justice.” Under that code, the duty to obey extends only to lawful orders.
These words look unremarkable. Their weight shows only by contrast. Soldiers of the Weimar Republic once swore loyalty to the Reich constitution. In August 1934, on the day Hindenburg died, they were required to swear a new oath: “unconditional obedience” to Adolf Hitler himself. Between a constitution and a man, there was only the text of an oath.
So as I understand it, America’s Rubicon is not first of all on a map. It is in one word of that oath: “Constitution.” That word replaced loyalty to a person with loyalty to a set of rules.
But this line has to be seen from both sides. Civilian control of the military is itself a republican principle. A president may replace generals, change military policy and direct the armed forces to carry out lawful missions; officers may not refuse to obey simply because they dislike the president. An army that decides for itself whom to obey has also crossed the Rubicon—just in the other direction.
The real line is narrow: obey lawful orders; refuse manifestly unlawful ones. What holds that line is not any general’s politics. It is thousands upon thousands of officers, prosecutors, judges and civil servants who, when they are asked to choose, still know where their loyalty lies.
2021: the people on the riverbank
The closest America has come to this river was after the 2020 election.
Trump lost the election and refused to accept it. His team filed more than sixty lawsuits across the states and lost nearly all of them, many before judges appointed by Republicans. Then the pressure turned to particular people. Georgia’s secretary of state, Brad Raffensperger, got a call from the president asking him to “find” 11,780 votes—exactly one more than Biden’s margin. He refused.
On January 6, 2021, the pressure fell on Vice President Mike Pence. Trump wanted him to refuse to count certain states’ electoral votes while presiding over the congressional certification. In a letter released that day, Pence wrote that “my oath to support and defend the Constitution constrains me from claiming unilateral authority to determine which electoral votes should be counted and which should not.” Hours later, some of the crowd that stormed the Capitol were chanting for Pence to be hanged. That night, Congress reconvened, and in the early hours of the next morning it completed the certification.
Six days later, every member of the Joint Chiefs of Staff signed a message to the entire force: “Any act to disrupt the Constitutional process is not only against our traditions, values, and oath; it is against the law.” The message also reminded the troops that on January 20, President-elect Biden would be sworn in as their 46th commander in chief.
Most of these people were not Trump’s political enemies. Raffensperger and Pence are Republicans; Pence had served Trump for four years. They did not need to oppose any of his policies. They only needed to refuse, within their own authority, to do what the law did not allow. That is exactly what Sulla’s officers had done: they were not necessarily against Sulla; they simply knew the line could not be crossed.
That time, the river held.
But the lesson of Rome is that holding once does not mean the water will stay as deep. What happens after a crossing attempt matters just as much.
After the 2024 election, the Justice Department, following its long-standing policy against prosecuting a sitting president, dropped the federal charges against Trump over his efforts to overturn the election. On January 20, 2025, the day he returned to the White House, Trump pardoned some 1,500 defendants connected to the Capitol attack and commuted the sentences of fourteen more. Those granted clemency included people convicted of assaulting police officers or of conspiring to stop the transfer of power.
The pardon power is expressly granted to the president by the Constitution, and supporters can reasonably argue that some defendants were sentenced too harshly. But the message these pardons sent went far beyond individual cases: those who tried to stop a lawful transfer of power can be forgiven for their political loyalty. For the next person standing on the riverbank, the cost of crossing just got lower.
Someone is testing the depth
Nothing that has happened since 2025 is a Caesar-style crossing. The president has not ordered troops to surround Congress, nor has he announced that elections are cancelled. That is precisely why these events deserve a closer look. Taken one by one, each can be given a legal justification. Taken together, they look like someone probing the river, spot by spot, to see how deep it is.
The first spot is the military and the cities. In June 2025, over the objections of California’s governor, Trump federalized the California National Guard and sent it to Los Angeles, followed by Marines. Similar deployments or attempts followed in Washington, Memphis, Portland and Chicago. On September 30, hundreds of generals and admirals were summoned on short notice to Quantico, Virginia. Trump told them that America was facing an “invasion from within,” and that some “dangerous cities” should be used as training grounds for the military. Reporters in the room described the generals as largely silent. In January 2026, as protests against federal immigration enforcement spread in Minnesota, Trump threatened to invoke the Insurrection Act and send in regular troops, then said the next day there was no reason to do so “right now.”
The question is not whether the military may ever appear on American soil. In disasters, in riots, when federal law truly cannot be enforced, the law allows it. The question is this: when a president describes his domestic political opponents as enemies who simply don’t wear uniforms, he is quietly moving the line in soldiers’ minds. The “domestic enemies” their oath tells them to defend against—are they people who would subvert the Constitution, or people who oppose the president?
The second spot is the military’s lawyers. On February 21, 2025, Trump removed the chairman of the Joint Chiefs of Staff; the same day, Defense Secretary Pete Hegseth dismissed the Judge Advocates General of the Army, Navy and Air Force. These three were the military’s most senior legal advisers, and part of their job is precisely to tell commanders whether an order is lawful. Hegseth later explained that he did not want military lawyers to be “roadblocks to orders that are given by a commander in chief.”
The president has the authority to replace his generals; that in itself is not illegal. But when Sulla marched on Rome, it was his officers who walked away. Today, the people whose job is to tell officers which orders are unlawful were the first to be replaced.
The third spot is the oath itself. In November 2025, six Democratic members of Congress with military or intelligence backgrounds released a video reminding service members that they can refuse illegal orders. That sentence does little more than restate the oath and the military code. Critics can argue that the video never said which orders were illegal, and that its timing and intent were political—that is a fair debate. But Trump’s response was to call it “seditious behavior, punishable by death.” In January 2026, Hegseth issued a formal letter of censure to one of them, Senator Mark Kelly, and opened proceedings to reduce the retired Navy captain’s rank and pension.
I consider this one of the most alarming moments of the past two years. It was not an order to the military to do something illegal. It made reminding the military not to do anything illegal into a dangerous act. If even restating the oath carries a price, what will the next officer who receives an unlawful order think? To make a river useless, you do not have to cross it. You only have to make people afraid to say where it is.
The fourth spot is the prosecutors. In September 2025, Trump publicly named his targets on social media and demanded that the attorney general go after former FBI director James Comey, New York attorney general Letitia James and others. The U.S. attorney for the Eastern District of Virginia resigned under pressure, and a White House aide with no prosecutorial experience was installed in his place. Within weeks she had obtained indictments against both.
Here I have to be honest about an objection. Trump’s supporters will say: in 2023, Trump himself was criminally indicted four times—wasn’t that political prosecution? I do not think the two can simply be equated. Those cases were brought by a special counsel and by local prosecutors, went through grand juries, and in one of them a trial jury returned a guilty verdict. In this case, the president first named his targets in public, then replaced a prosecutor who would not bring charges.
Yet the objection points to a real danger, and it is exactly the lesson Caesar left behind: once both sides believe that losing power means being dragged into court by the other, no one will be willing to hand over power peacefully. So whether a prosecution crosses the line depends not on who the defendant is, but on the process itself. Is the decision to prosecute driven by evidence and independent prosecutorial judgment, or by the president’s wishes?
But the river is still there
Over these two years, many people have also taken their stand on the riverbank—and not all of them are Trump’s opponents.
The state governments of Illinois, California and Oregon took the federal government to court. Here, federalism is not an abstract principle from a textbook but a real levee: when the president wants to turn a state’s militia into a federal army, the governor can say no, and go to a judge.
On December 23, 2025, the Supreme Court turned down the administration’s emergency application in the Chicago case. Its unsigned opinion hinged on a single phrase. The statute the president had invoked lets him federalize the Guard only if he is “unable with the regular forces” to execute federal law, and the Court read “regular forces” as most likely meaning the military itself. Because the military may lawfully enforce the law only in “exceptional” circumstances, the statute is confined to those circumstances as well. That judgment came from a court with a conservative majority. Before it, four lower-court judges had already ruled against the deployment, finding that conditions on the ground did not justify sending in the Guard. Two of the four had been appointed by Democratic presidents and two by Republicans; one of the appellate judges was Trump’s own appointee. The appeals panel put it bluntly: “political opposition is not rebellion.”
The cases against Comey and James were dismissed by a federal judge in November 2025, on the ground that the interim prosecutor’s appointment was itself unlawful. The Justice Department then took the case against James back to a grand jury twice, and was turned down both times. A grand jury is made up of sixteen to twenty-three ordinary citizens, and its refusal to indict at a prosecutor’s request is extremely rare in American practice. The Justice Department is still appealing; the Fourth Circuit heard arguments in mid-September and, as of this writing, has not ruled.
Mark Kelly’s case was halted in February 2026 by a federal judge appointed by George W. Bush, who found that the Pentagon had violated Kelly’s freedom of speech and threatened the constitutional rights of millions of military retirees. Hegseth said the ruling would be “immediately appealed.”
The military’s top brass have not stayed silent to the end, either. In late August 2026, replying to a letter from Senator Elissa Slotkin, Joint Chiefs Chairman Gen. Dan Caine wrote that the military had no plans to send federal troops or federalized National Guard members to polling places, and no plans to seize ballots or voting machines. He added one more sentence: “I have neither received nor anticipate receiving any unlawful order” concerning the military’s role in the midterm elections.
Reading that sentence, I had mixed feelings. On one hand, here was a general hand-picked by Trump writing the words “unlawful order” in a public letter—he knows where the line is. On the other hand, the country with the world’s oldest written constitution now needs its highest-ranking officer to promise, in black and white, that troops will not show up where its citizens vote.
The river is still there. But it is there not because it is simply part of the landscape, but because someone has always been guarding it.
The Romans later forgot where the river was
Back to the question I started with: where is America’s Rubicon?
My answer is that it is not in the White House, nor on Capitol Hill, nor in the result of any one election. It is in one word of the oath—“Constitution”—and in every particular person who is asked to make a choice: an officer who receives an order, a prosecutor told to bring charges, a secretary of state asked to “find” votes, an ordinary citizen sitting on a grand jury.
So the real question may not be “Will the president cross the river?” but “Is the river still there?”
A crossing is a dramatic moment; everyone can see it, and it can be resisted. The more dangerous possibility is a different one: no one crosses, and the river simply dries up. The people whose job is to say “this is unlawful” are replaced. Those who remind soldiers of their oath are called seditious. Who gets prosecuted depends on whose name the president posts. Those who tried to stop a transfer of power are pardoned. Every step has a legal basis; every step can be explained away as an exception. Until one day a manifestly unlawful order really is given, and the person who receives it looks around and finds that no one can say any longer where the line is.
This is not only a metaphor. The location of the Rubicon really was forgotten.
After the Republic fell, the little river lost its legal meaning, and with it, slowly, any reason to be remembered. Its course shifted, place names changed, and several small rivers in northeastern Italy were each held to be the one; the dispute went on for centuries. In 1933, Mussolini’s government officially designated one of them, the Fiumicino, as the Rubicon, and the town on its bank was renamed to match. Later scholars found some evidence for the choice, but the debate has never been fully settled.
Whenever I think about this, I feel an irony I can’t quite put into words: a dictator decided for the Romans where their Rubicon was.
And in Rome itself, nearly all the names survived. Augustus claimed he had handed the Republic back to the Senate and the people of Rome. The Senate still met; consuls were still elected. Only everyone knew that the real decisions were made elsewhere. The names were all still there. The river was gone.
I am not American. But I have always thought that what is most remarkable about the United States is not its wealth or its power, but that it turned “no one is above the law” into a system that has worked for more than two centuries. That system has no walls, just as the Rubicon had none. How deep it runs depends on whether those who hold power truly believe in it—not on the day they take their oath, but on the day believing in it comes at a price.
As long as enough people remember where the river is, it remains a border. By the time those in power have to tell everyone where the river is, the republic is already gone.
Notes: Caesar’s words at Pharsalus and Cato’s threat to prosecute him are in Suetonius, Life of the Deified Julius, 30; the desertion of Sulla’s officers is in Appian, Civil Wars, 1.57; Caesar’s speech to his soldiers about the tribunes is in Caesar, The Civil War, 1.7; Augustus’s claim to have handed back the Republic is in the Res Gestae Divi Augusti, 34. Caesar’s words are translated by the author directly from Suetonius’s Latin; the other translations are also the author’s own.