Australia and other US allies tuning in to the presidential debate would have been reassured by Hillary Clinton's performance, but puzzled by the "international man of mystery" Donald Trump, experts say.
The presidential rivals touched on several foreign policy issues during the 90-minute debate including security and trade alliances, nuclear weapons and trouble spots such as North Korea and Iran.
While Mrs Clinton insisted it would be business as usual in terms of security alliances if she wins power in November, Mr Trump said America couldn't afford to be the "policeman of the world" if its friends didn't pay for its defence support.
Professor David Ribar, deputy director of the Melbourne Institute of Applied Economic and Social Research, said Mrs Clinton's message focused on a continuation of fundamental US policies and a cooperative framework.
"As for Mr Trump, he's an international man of mystery. We have no idea what we would end up getting," he told AAP.
"He talks a good game but it's hard to see how he would make good on many things he described. He's unwilling to say how he would carry his policies out."
Mrs Clinton highlighted her diplomatic experience as secretary of state and noted how many world leaders were worried about the prospect of a Trump presidency.
The US would honour its treaties if she became president, she said, adding that she intends to "be a leader people can count on here and around the world".
Mr Trump hit back, saying he had better judgement and more common sense than his rival, accusing Mrs Clinton and the Obama administration of leaving the Middle East in "a mess" and creating a vacuum for ISIS to emerge.
On the NATO security alliance, he said the 28-member "aren't paying their fair share" for the "tremendous" protection provided by America.
Political science Professor Leonie Huddy, an Australian-born, US-based academic, said Mr Trump's comments indicated NATO allies could be facing a pretty big bill if he wins office.
"Given Obama's pivot to Asia it also raises questions about what would be happening in the Pacific under a Trump presidency," said Prof Huddy, who is based at the State University of New York at Stony Brook.
On trade, both candidates insisted they opposed the controversial Trans-Pacific Partnership, of which Australia is one of 12 signatory countries.
Mr Trump said the US had to renegotiate a swathe of trade deals because they had cost thousands of American jobs, before seizing on Mrs Clinton's initial support for the TPP before her recent change of heart.
Dr Raymond Orr, of the Melbourne School of Government, said Mr Trump's comments on trade were probably the strongest he made during the debate as they tapped into widespread resentment about job losses among America's working class.
But while millions of people around the world tuned in to the debate, Dr Orr said whoever was judged the winner would probably only pick up a few points in public opinion polls.
"It doesn't matter much unless it's a total disaster, but it's good entertainment," he said.
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