In the face off televised live and watched by millions on Thursday, an aggressive Kerry accused the president of a "colossal error of judgment" in invading Iraq, but Bush insisted the world was a much safer place without Saddam Hussein.
"This president has made - I regret to say - a colossal error of judgment, and judgment is what we look for in the president of the United States," Kerry said in arguing that Bush rushed to war without earning allied support or planning for peace.
Bush shot back, saying "the world is safer without Saddam Hussein" and said Kerry was sending the wrong signal to US troops and the nation's enemies.
"I don't see how you can lead this country to succeed in Iraq if you say 'wrong war, wrong time, wrong place'. What message does that send our troops, what message does that send to our allies, what message does that send to the Iraqis?" Bush said.
Bush defended the Iraq war but
Kerry said it was bad judgment
Bush and Kerry locked horns over who could best protect America, the need for the US-led invasion of Iraq, the threats facing America and how to lead the country to counter terrorism.
The 90-minute televised session on the University of Miami campus gave voters their first chance to compare the candidates directly in an encounter that both camps expected would be crucial in the November presidential polls.
Bush expectedly accused Kerry of flip-flopping and changing positions on Iraq too often.
Kerry denied the charge, insisting "people know where I stand. People out there listening know what I believe".
He acknowledged that he had not always been artful in his stance on Iraq. He voted to authorise that war but has criticised its conduct, and in August said he would vote to authorise it again.
"I made a mistake in how I talk about the war, but the president made a mistake in invading Iraq. Which is worse?" Kerry asked.
"I think we need a president who has the credibility to bring the allies back to the table and to do what's necessary to make it so America isn't doing this alone," he said.
"Iraq was not even close the centre of the war on terror before the president invaded it," Kerry said. "While it is important to be strong and resolute, we also have to be smart," he argued.
Both the president and his challenger agreed that the spread of nuclear weapons was the biggest threat facing the US, but they disagreed over how to combat it.
Both agreed that nuclear weapons
posed the biggest threat to the US
Kerry accused Bush of doing little to keep nuclear materials in the former Soviet Union secure and prevent North Korea from acquiring nuclear weapons.
"The president actually cut the money for it. You have to put the money into it and the funding and the leadership," he said.
He also accused Bush of sending mixed messages by spending hundreds of millions of dollars to research bunker-busting nuclear weapons in the US while telling other countries not to pursue nuclear arms.
"We are telling other people, you can't have nuclear weapons, but we are pursuing a new nuclear weapon we might even contemplate using," he said. He promised to shut down the programme if elected.