On Monday, the court’s conservative majority ruled that any “official acts” a president takes — even beyond the office’s “core constitutional functions” — would enjoy “presumptive immunity” from prosecution.

But in the US, the president also serves as the head of the military, and experts say Monday’s decision could further strengthen a culture of impunity for actions taken abroad.

Samuel Moyn, a professor of law and history at Yale University, said the decision erodes the few guardrails left to govern US foreign policy.

Already, the US Congress gives presidents wide latitude to take actions overseas, and the country refuses to recognise the authority of bodies like the International Criminal Court (ICC).

“There was already a large consensus among conservative and liberal elites that a US president should never be restricted by international courts outside of the country,” Moyn told Al Jazeera.

“What was extraordinary about Monday’s ruling is that it seems to take that attitude and import it — to apply it to courts inside the country as well as outside.”

A powerful shield

The ruling came about after former President Donald Trump asserted far-reaching claims to presidential immunity, as he tried to dodge four separate criminal indictments in US courts.

“Trump asserts a far broader immunity than the limited one we have recognized,” the court majority explained in its opinion.

Still, it held that any act deemed an “official” part of the presidency could be shielded from criminal charges.

But even the court acknowledged that this could herald “king”-like executive powers with few criminal constraints. Foreign policy was one area the dissenting justices highlighted.

“From this day forward, Presidents of tomorrow will be free to exercise the Commander-in-Chief powers, the foreign-affairs powers, and all the vast law enforcement powers enshrined in [the US Constitution] however they please — including in ways that Congress has deemed criminal,” Justice Sonia Sotomayor wrote in her dissent.

Already, the Supreme Court had established legal precedent in the 1980s that gave presidents “absolute immunity” from civil damages for their conduct while in office.

That put presidential actions out of reach of laws like the Alien Tort Statute, which allows foreign nationals to pursue human rights violations in US civil courts.

An evolving executive

But experts say that Monday’s decision continues a trend of giving ever-greater power to the executive branch over matters of foreign affairs.

Under the US Constitution, the president and Congress share the powers to shape foreign policy. But the legislative branch has ceded ground to the presidency, particularly in periods of national emergency such as the Cold War and the attacks on September 11, 2001.

While it is difficult to pinpoint a single moment when authority over foreign affairs became concentrated in the White House, overseas conflicts helped strengthen what some critics call “the imperial presidency”.

Coined in 1973, that term describes a perception among some historians that the US presidency has exceeded its constitutionally mandated powers, particularly when it comes to overseas actions like warfare.

The US Constitution gives Congress the exclusive authority to declare war, but the last time it formally did so was in World War II.

The Cold War, meanwhile, saw an ever-growing number of defence and intelligence bodies take shape under executive control. That period saw institutions like the Central Intelligence Agency (CIA) crop up in 1947, and the National Security Agency in 1952.

Experts say these defence and intelligence groups helped the US wage a global campaign to expand its influence, sometimes through clandestine operations and even torture and assassination.

At times, after revelations of abuses, the legislative branch tried to claw back influence over US foreign policy.

One example came in the early 1970s, when an emboldened Congress barred President Richard Nixon from sending weapons to the government of Pakistan after a campaign of brutal repression came to light. It also moved to rein in the president’s secretive military incursions into Cambodia during the Vietnam War.

But such stabs at oversight proved to be the exception rather than the rule, and presidents have historically faced few consequences for overseas actions that could constitute violations of international and domestic law.

Nixon, for instance, continued to send weapons to Pakistan, albeit through proxies like Jordan, in defiance of Congress’s sanctions.

‘Rule of law at home but none abroad’

The country’s appetite for reining in the White House continued to diminish after the September 11 attacks, according to experts like Moyn.

After President George W Bush declared a global so-called “war on terror” in 2001, US presidents have carried out military operations in nearly 80 countries.

Critics say perceived enemies have been captured and tortured in the name of national security, including at CIA black sites and the detention centre in Guantanamo Bay, Cuba.

The 2014 drone strike that killed the Yemeni American Muslim leader Anwar al-Awlaki also raised uncomfortable questions about whether a sitting president should be able to execute a US citizen without a trial.

US courts have mostly declined to weigh in on such matters, Moyn explained.

He said that presidents have largely been given a “permission slip” to take drastic overseas actions, with government legal advisers finding creative ways to give human rights abuses the imprimatur of legal compliance.

President Barack Obama, for instance, signed an executive order to end the torture programme launched under Bush.

But while Obama admitted the US “tortured some folks”, he declined to prosecute those responsible under the previous administration, calling on the country to “look forward, not back”.

The US has also been hostile to international efforts to ensure criminal acts are prosecuted.

During the Bush administration, Congress went so far as to pass a bill authorising the invasion of The Hague in the event that Americans were ever placed on trial before the ICC.

“A lot of Americans have come to see it as normal to have the rule of law at home and none abroad,” said Moyn. “You can’t lament the erosion of the rule of law domestically when you were cheering on its abolition overseas.”

‘A dictator on day one’

Still, some experts worry that the Court’s decision could amplify patterns of impunity that have long-defined executive power overseas.

Trump, the all-but-certain Republican nominee for the White House in 2024, has pledged to wield executive power to crush his political enemies.

“Under this Court’s opinion, the only thing that will stop a president from abusing their power is their own sense of restraint and people in the executive branch who may not follow their orders,” said Chris Edelson, an assistant professor of government at American University and author of Power Without Constraint: The Post 9/11 Presidency and National Security.

Last year, for instance, Trump said that he would be a dictator, if only on his first day in office, in order to “close the border” with Mexico.

“Presidents like Bush and Obama weren’t dictators. But once you give unchecked power to one president, all presidents have that power. And down the line, you might have a president who does want to be a dictator,” said Edelson.

“In fact, we face the imminent prospect that a would-be dictator could take office if Trump wins the election this fall.”