In 1957 nine African-American pupils attempted to register at Little Rock Central High School in Arkansas. The previously white-only school was belatedly complying with a Supreme Court decision, Brown v Board of Education, issued three years before that ordered the desegregation of schools. Yet, after braving a mob of some 300 who heckled and hissed, the students found their entry barred by members of the Arkansas National Guard, summoned by the segregationist governor, Orval Faubus. The president, Dwight Eisenhower, soon resolved the crisis by federalising the entire Arkansas National Guard, wresting it from the control of the governor. The very guardsmen who had been ordered to prevent the pupils’ attendance now escorted them into the building.

Ordinarily, state National Guards are under the control of governors. Most of their members hold civilian jobs for much of the year, but can be mobilised for emergencies such as natural disasters or unrest. They are the modern-day versions of the colonial state militias; their role is enshrined in the constitution. Article I assigns Congress the power “to provide for calling forth the Militia to execute the Laws of the Union, suppress Insurrections and repel Invasions”. Yet they also are reservists for the professional military services and can be commanded by the president and deployed abroad when needed.

In the two decades after Little Rock, state governors or the president mobilised the Guard regularly, often in response to unrest, though for other purposes too. In March 1970 Richard Nixon, then the president, ordered the deployment of almost 29,000 soldiers to distribute letters amid a postal workers strike. It was not a particularly effective solution, and soon a bargain was struck to get the mailmen back to work.

One infamous and ugly use of the National Guard came in May 1970, to put down anti-Vietnam War student demonstrations. President Richard Nixon had announced that America would expand the war in Vietnam to Cambodia. A day later, students at Kent State University in Ohio started protesting. These demonstrations soon escalated into clashes with the police. James Rhodes, Ohio’s governor, called the National Guard in. The guardsmen instructed the protesters to disperse. They did not. Soldiers threw tear-gas; protesters chucked rocks. Eventually the guardsmen opened fire, killing four students. Their deaths prompted large protests and student walk-outs across the country, though in a Gallup poll shortly after the shootings more than half of respondents said the students were to blame, compared with barely a tenth who held the National Guard responsible.

After this, the public and politicians soured on the idea of calling in the Guard. The next time they were deployed amid unrest in the continental United States was over two decades later. In April 1992 four police officers were acquitted of the beating of Rodney King, a black man, despite it having been caught on film. Enraged Angelenos took to the streets in protest, leading to several days of rioting and 63 deaths. Pete Wilson, California’s governor, deployed the National Guard. Several days later, George H.W. Bush sent several thousand soldiers, marines and federal police, too. More recently, in 2020, dozens of states called in the Guard during nationwide protests against the killing of George Floyd, another black man, by police.

The very public spat between Mr Trump and Mr Newsom has prompted both sides to exaggerate the purpose of the mobilisation. America’s armed forces are not actually carrying out deportations. A longstanding principle of English common law is that troops are prohibited from carrying out domestic law-enforcement operations. In America this was formalised in the Posse Comitatus Act of 1878, which bans the practice unless specifically authorised by Congress or the constitution.

So far, Mr Trump has authorised a limited mission to the 4,000 members of the National Guard under his command that does not involve making arrests. They are to protect federal buildings and personnel. Department of Justice lawyers have long argued that presidents hold this so-called “protective power”. For Mr Trump to assign the guardsmen and recently deployed marines tasks of keeping the peace, he would have to invoke another authority like the Insurrection Act. This law, enacted in 1807, does not require the president to obtain the consent of a state’s governors in order to deploy troops there when “unlawful obstructions, combinations or assemblages, or rebellion against the authority of the United States, make it impractical to enforce the laws”. In such cases, the president can use troops “as he considers necessary to enforce those laws or to suppress the rebellion”.

What is different about Mr Trump’s deployment is that it seems to have inflamed the protesters in the name of keeping the peace. The state of California has filed suit claiming that the president’s actions were illegal. Courts are generally reluctant to restrict presidential actions premised on national security, says Chris Mirasola, a law professor at the University of Houston. But even if they did, Mr Trump could easily invoke the Insurrection Act, which would enable him to go further than he already has. Any ensuing violence would be treated as post-hoc justification for mustering troops in the first place. In such a case, it would also be harder for federal judges to intervene, for instance by declaring that the conditions of “insurrection” or “rebellion” had not been met.

Neither Mr Newsom and Mr Trump seem prepared to back down. There is, unfortunately, political upside in a protracted showdown for both men: Mr Newsom is plainly angling to be the next president and picking fights with the current administration burnishes his resistance credentials. Mr Trump would rather fight with his Democratic antagonists over immigration enforcement and imposing law-and-order, which he believes the silent majority backs him on, than reckon with his chaotic tariff policy or faltering diplomatic efforts. The president has extensive powers—ones that he is expected to exercise judiciously, in the interest of the country rather than himself or his party. The question is not whether Mr Trump is acting legally—he probably is—but whether or not he has the self-restraint to act rightly. ■