President Joe Biden, right, listens as Republican presidential candidate former President Donald Trump speaks during a presidential debate hosted by CNN in Atlanta, U.S., June 27. AP-Yonhap

President Joe Biden and his predecessor Donald Trump locked horns over defense policy, immigration, inflation, Russia's war in Ukraine and other hot-button issues in this year's first general election debate on Thursday, lambasting each other as the "worst president" in America's history.

Hosted by CNN, the 90-minute rancorous debate took place without an audience in the battleground state of Georgia more than four months ahead of the Nov. 5 presidential election. Much was at stake for both Biden and Trump, who have been in a tight race according to various nationwide polls.

Calling Biden's defense policy "insane," Trump cited tensions with North Korea, Russia and China and accused the incumbent president of bringing the United States "closer to World War III."

"His military policies are insane. These wars will never end with him. He will drive us into World War III," he said. "President Xi of China and Kim Jong-un of North Korea ... Putin. They don't respect him. They don't fear him."

In a hoarse voice, Biden hit back, reminding the public of Trump's controversial stump speech in February, in which Trump said that if reelected, he would "encourage" Russia to "do whatever the hell they want" to "delinquent" NATO members that fail to meet their defense spending commitments.

"You want a World War III? ... Just do what you want," he said.

"(If) you want to have war, just let Putin go ahead and take Kyiv, make sure they move on and see what happens in Poland, Hungary and other places along that border," he added.

Biden criticized Trump for casting the U.S. as a country that is "failing" due to his policy.

"We are the most progressive country in the world ... We are the strongest country in the world. We are a country in the world who keeps our word and everybody trusts us," he said.

Amid Trump's attack on Biden's economic scorecards, Biden highlighted his policy achievements, including attracting investment from foreign companies, like Samsung Electronics.

"I went to South Korea. I convinced Samsung to invest billions of dollars here in the U.S.," he said.

A bitter clash came when a question about immigration emerged. Trump accused Biden of allowing "millions of people from jails and mental institutions" to come into the U.S. and "destroy our country."

As expected, Trump also claimed that the U.S. has the "largest number of terrorists" coming in from all around the world.

"We have a border that's most dangerous place anywhere in the world and he opened it up," Trump said. "These killers are coming into our country and they are raping and killing women."

Migrants watch the first debate between U.S. President Joe Biden and former U.S. President and Republican presidential candidate Donald Trump in Tijuana, Mexico, June 27. Reuters-Yonhap

Biden stressed his administration's effort to get a bipartisan agreement for border security, while accentuating what happened during Trump's presidency.

"When he was president, he was separating babies from their mothers, putting them in the cages, making sure that the families are separated," he said. "That is not the right way to go."

On inflation, tension between the two intensified.

Trump argued that inflation at present is "absolutely killing us."

"The only jobs (Biden) created are for illegal immigrants and bounced-back jobs — bounced back from COVID," he said. "He's done a poor job and inflation is killing our country."

The two sides also touched on sensitive issues. Biden pointed out that Trump was the only convicted felon at the stage. Trump raised an issue with Biden's son, Hunter.

They also pummeled each other, excoriating each other as the worst president in the U.S. history.

"Without question, the worst president in the history of our country," Trump said.

Biden said that a vote by presidential historians showed that Trump was "the worst in all of American history."

"That's a fact. That's not conjecture," he said.

It was the first general election debate pitting a sitting president against a former president.

It was also the earliest presidential debate given that such debates typically take place in late September and October. Biden's campaign wanted earlier debates for voters to assess candidates before the start of early voting.

The next presidential debate is scheduled for Sept. 10. (Yonhap)