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.”