Allies of the United States had already been looking at the country’s upcoming election with anxiety. Now, with the United States Supreme Court granting an unprecedented expansion of executive power by giving presidents legal immunity, analysts in some of those countries are even more concerned about the reliability of American power.

Across Asia and Europe, where allied leaders have grown accustomed to dealing with threats from authoritarian leaders in Russia, North Korea and China, the idea that they might also have to deal with an unfettered American president is an unsettling prospect.

“If the U.S. president is free from the restrictions of criminal law, if he has that level of criminal immunity, the other leaders of the allied nations cannot trust the U.S.,” said Keigo Komamura, a professor of law at Keio University in Tokyo. “We cannot maintain a stable national security relationship.”

Mr. Komamura added that the Supreme Court’s decision now gave the perception of an American president who can operate above the law. “This may be rude to the U.S., but it is not that different from Xi Jinping in China,” he said. “The rule of law has become the rule of power.”

Though some give limited immunity to leaders while in office, Japan, South Korea, Australia and Great Britain — among the United States’ closest allies in the world — offer nothing like the sweeping protections the Supreme Court appears to have granted in its ruling this week.

The court’s decision to give the president immunity from criminal prosecution for official conduct — which was itself vaguely defined by the court — was “out of line with global norms,” said Rosalind Dixon, a professor of law at the University of New South Wales in Sydney. “I think that what is occurring in the United States in terms of the court’s ruling and the presidential election should be of grave concern to all of America’s allies.”

In South Korea, political leaders essentially have no legal protections from criminal prosecution once they are out of office — and the president is limited to a single term. Four of the past eight former presidents have been convicted and imprisoned after leaving office for corruption and other crimes they committed before and while they were in office.

“I think many Koreans are proud of the fact that no one is above the law, even the president,” said Ramon Pacheco Pardo, a professor of international relations at King’s College London and the chair of Korean studies at the Brussels School of Governance at Vrije Universiteit. “But in the U.S., it appears that presidents are created differently from the rest of the people.”

Still, the frequency of criminal indictments of officials in South Korea has contributed to increasing political polarization, with some supporting the punishments as acts of justice and others viewing them as little more than political revenge orchestrated by a new president.

While in office, presidents of South Korea have immunity from criminal indictment except in cases of “insurrection or treason.” Such a clause was pointedly not included in the U.S. Supreme Court’s decision, which ruled that former President Donald J. Trump is entitled to immunity from prosecution on charges that he tried to overturn the result of the 2020 presidential election.

In Japan, the Constitution grants all members of the Diet, as Japan’s Parliament is known, immunity from arrest while in office, but not from criminal prosecution, Mr. Komamura said. The prime minister, who must be a member of Parliament, is covered under this clause.

One of the biggest scandals of the 1970s in Japan was when former Prime Minister Kakuei Tanaka was indicted on bribery charges for having accepted $1.6 million from Lockheed to arrange the purchase of aircraft by All Nippon Airways, Japan’s largest airline.

Even in countries where there is some immunity for political leaders, it is usually more narrowly defined. In the United Kingdom, where members of Parliament broadly enjoy legal protections from prosecution for political speech, they are not immune from the criminal laws that govern the public.

The police fined former Prime Minister Boris Johnson, for example, while he was still in office for attending a lockdown party at Downing Street that broke coronavirus laws that his own cabinet had instituted during the pandemic.

Even where legal immunity is more strictly defined, though, the laws may not be as big a factor as political culture.

In Malaysia, although executive immunity is not as sweeping as the U.S. Supreme Court just granted to presidents, a culture of impunity has meant that few leaders are taken to court despite widespread corruption.

For years, former Prime Minister Najib Razak escaped a criminal conviction for a multibillion-dollar corruption scandal because of his control of the country’s courts and media.

After the opposition came to power in 2018, he was found guilty in 2020 on seven corruption counts and sentenced to up to 12 years in prison. Earlier this year, however, his sentence was halved, and his fine was cut to a quarter of the original amount by the country’s pardon board. There has been widespread speculation that he was about to receive royal clemency from the king.

“Maybe Trump can get a royal pardon like his good friend Najib in Malaysia,” posted one X user on Monday.

Whether legal prosecutions can derail politicians determined to stay in office is another question.

In Israel, all members of Parliament, including the prime minister, are subject to absolute immunity from prosecution of acts committed while performing their official duties. It is a protection not unlike that defined by the U.S. Supreme Court ruling.

That has not prevented prosecutions. Although Israel’s prime minister, Benjamin Netanyahu, was indicted on charges of bribery, fraud and breach of trust nearly five years ago, he has maneuvered tenaciously to stay in office. Before the war in Gaza, Mr. Netanyahu, whose corruption trial is ongoing, tried to expand his powers over the country’s courts, triggering mass protests in Israel.

In all that, he has departed from the precedent set by a predecessor, Ehud Olmert, who stepped down when he was mired in corruption investigations.

Adam Shinar, a professor of law at Reichman University in Tel Aviv, said that the Supreme Court’s ruling basically introduced in the United States the same kind of immunity that Israeli leaders have had since 1951. But he said that American presidents have enjoyed de facto immunity for decades.

“Nobody has ever talked about prosecuting them for things after they left office,” said Mr. Shinar. The closest anyone came was discussions about whether Richard Nixon would be prosecuted for the Watergate scandal, but his successor, Gerald Ford, pardoned him before any trial could take place.

The new U.S. court ruling has taken on particular urgency abroad in large part because of the prospect that Mr. Trump could again become president.

Mr. Shinar said that because of Mr. Trump’s disregard for legal or political norms, and the widening political divide and basic distrust in American government, the reaction to the Supreme Court’s ruling is more dramatic than it might have been in another era.

He added: “But if we have declining trust in our political institutions at the same time that there is growing immunity for our politicians, there’s a problem with that.”