While the first U.S. presidential debate of the 2024 race dwelled little on foreign policy, a shaky performance by 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 also 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 Korean, 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.

He 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 November 5 election.

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

Peter Lee, research fellow at the Asan Institute for Policy Studies in Seoul, said the debate 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 defence spending.

Lee Jae-il, analyst at Eugene Investment & Securities, said that companies dependent on U.S. markets will also be wary because "the previous Trump administration had a myriad of tariffs-related policies in the past".

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.

"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, after attending a workshop titled "Trump 2.0" that included Australian officials.

"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." (Additional reporting by Hyunsu Yim in Seoul and Lews Jackson in Sydney; Writing by John Geddie; Editing by Miral Fahmy)