While the first U.S. presidential debate of the 2024 race dwelled little on foreign policy, a shaky performance by President Joe Biden will have America's allies steeling for the return of Donald Trump, analysts say.

Biden's supporters had hoped the debate would erase worries that he was too old to serve, but several lawmakers, analysts and investors said the event had given Trump a boost.

"Mr. Trump didn't win but Mr. Biden might have imploded," said Kunihiko Miyake, a former Japanese diplomat and now research director at the Canon Institute for Global Studies, a think tank.

"Unlike eight years ago, we are much more prepared, as are other European and Asian allies. Still, Mr. Trump is unpredictable."

For Japan and South Korea, among the closest U.S. allies in Asia, relations with Trump's administration were at times strained by his demands for more payments towards military assistance as well as trade tensions.

"The biggest question for Japan would be whether Trump will truly value and maintain the security alliance," said Takashi Kawakami, a professor at Japan's Takushoku University in Tokyo.

Peter Lee, research fellow at the Asan Institute for Policy Studies in Seoul, said the debate had put into "much more stark relief" the possibility of a second Trump administration. Lee said he expected Trump to be "very tough" second time around in pressuring allies to up their defense spending.

Trump also started a tariff war with China, the world's second largest economy, and has floated tariffs of 60% or higher on all Chinese goods if he wins the Nov. 5 election.

Overseas firms dependent on U.S. markets, such as automakers, would also be wary of the heightened possibility of Trump's return given the "myriad" of tariff-related policies he imposed during his previous term, said Lee Jae-il, analyst at Eugene Investment & Securities.

"Trump, like a trade war maniac, might not just target China but impose tariffs against other countries as well under the concept of American exceptionalism," added Stephen Lee, chief economist at Meritz Securities in Seoul.

In Europe, Trump's criticisms of the North Atlantic Treaty Organisation and demands that other members pay more dominated his previous administration. His skepticism towards NATO is causing further anxiety this time, as Russian's war in Ukraine has brought conflict to the bloc's doorstep.

German Chancellor Olaf Scholz previously cheered on Biden's prospects for reelection. But on Friday a senior defense figure in the ruling coalition lamented Biden's performance and urged Democrats to find another candidate.

"The fact that a man like Trump could become president again because the Democrats are unable to put up a strong candidate against him would be a historic tragedy that the whole world would feel," Marie-Agnes Strack-Zimmermann, of the liberal FDP party, told the Rheinische Post paper.

A spokesperson for Scholz did not comment on the specifics of the debate, but stressed the chancellor valued Biden highly and that Scholz had never spoken to Trump as their terms did not overlap.

During the debate, Trump accused Biden of not standing up to China on trade. He also said China's Xi Jinping, North Korea's Kim Jong Un and Russia's Vladimir Putin "don't respect" Biden and that he was driving the country "into World War Three."

Biden retorted by saying Trump's tariff proposals would result in higher costs for American consumers, and that he "cuddles up" to the likes of Kim and Putin.

Putin has said it made little difference to Russia who was in the White House, and on Friday the Kremlin declined comment on what it said was an internal U.S. matter.

U.S. allies including Japan and Germany have in any case started laying the groundwork for a possible Trump return in recent months.

"The possibility of a Trump comeback may have increased. High tariffs and support for Ukraine will also bring about major changes," said Kazuhiro Maeshima from the Sophia University in Tokyo.

"The Japanese government has also been anticipating various developments in the event of a Trump and has been gradually taking steps to deal with them, including making contact with people close to Trump."

In Sydney, several Australian officials and experts had attended a workshop titled "Trump 2.0" as the debate was aired.

"The overwhelming feeling from today is that it was a disaster for Biden," said Peter Dean, a professor at the United States Studies Centre in Sydney who was at the workshop.

"The mood has changed considerably after the debate and the general view is that if you weren't preparing for a Trump 2.0 then that is the smart play and the smart move now."

Keir Starmer, the leader of Britain's Labour party and the frontrunner in an election due next week, was asked on BBC radio if he was concerned about Biden after the debate.

"I've got enough on my hands with our own election campaign at the moment...The relationship between the UK and the US is strong, it's historic, and obviously, it's above the individuals," he said.