OXFORD, Miss. — John McCain and Barack Obama came to their first debate with clear missions. McCain's was to paint his rival as naive and inexperienced, Obama's was both to prove McCain wrong on that front and to tag his rival as a participant in eight years of failed Bush administration policies at home and abroad.

Each rose to the challenge yesterday in a 90-minute debate at the University of Mississippi, forcefully scoring points on each other, sparkling at times, but neither emerged as the obvious winner except perhaps to their partisans.

With the country facing an unresolved financial crisis as big as any since the Great Depression and the two candidates running in a highly competitive race for the White House, this was obviously a debate with enormously high stakes for both McCain and Obama. By the time it was over, it was evident just how large the differences were between on many of the biggest national security issues that will await the next president — and some domestic ones as well.

After the tumult of the week in Washington and on Wall Street, it was questionable whether any event could compete in terms of drama, excitement and possibly significance. McCain's high-risk gamble of suspending most campaign activity and returning to Washington to inject himself into negotiations over an economic rescue package threatened either to delay the debate or, at a minimum, overshadow it.

For a time, it looked like neither was ready to engage the other. They wandered sometimes aimlessly through the first 30 minutes of the 90-minute session, which dealt not only with the financial crisis that threatens the economy — and nearly scuttled the debate when McCain, the Arizona Republican, initially said they should stay in Washington to deal with it — but with domestic issues ranging from budgetary earmarks to tax cuts to healthcare.

But when the debate turned to the announced topic — foreign policy and national security — they came alive. Their exchanges were lively, pointed and revealing.

An exchange on the Iraq war exposed fundamental differences on what each man thinks this election is about.

"The first question is whether we should have gone to war in the first place," said Obama, the Illinois Democrat. "Six years ago, I stood up and opposed this war at a time when it was politically risky to do so."

But McCain said the most important question for Americans was which of the two would end the war in the right way.

If he is president, McCain said, "We will succeed and our troops will come home and not in defeat. ... We will see a stable ally in the region."

Obama, 47, said the 72-year-old McCain was wrong on the invasion, McCain said Obama was wrong about the surge.

WAR BECOMES HOT TOPIC

The two presidential candidates stood behind identical wooden lecterns on stage at the performing arts center at Ole Miss for the first of three scheduled debates with less than six weeks remaining until Election Day.

McCain and Obama argued over Afghanistan and who knew better how to deal with the resurgence of the Taliban and al-Qaida. Obama said McCain and Bush had let Afghanistan slip backward because of their focus on Iraq. McCain said that the same surge policy he supported in Iraq is what will put Afghanistan right.

They squabbled over how to prod Pakistan to deal with terrorist breeding grounds inside their borders. Obama said the United States had wrongly coddled the government of former President Pervez Musharraf, while McCain said Obama failed to understand that Pakistan was a failed state when the former general took power.

At least once, McCain sought to depict his rival as naive on foreign policy. That was in response to Obama's statement that it might become necessary to send U.S. troops across the Pakistani border to pursue terrorists.

"You don't say that out loud," McCain retorted. "If you have to do things, you do things."

He also criticized Obama for having said he would sit down without precondition with Iranian President Mahmoud Ahmadinejad.

"So let me get this right. We sit down with Ahmadinejad, and he says, 'We're going to wipe Israel off the face of the Earth,' and we say, 'No, you're not'? Oh, please!"

But Obama argued that while McCain has resisted talks, many others have supported them and that isolating dangerous nations, as he said Bush has done, has only made things worse.

MAKING THEIR CASES

McCain gained strength as the debate wore on, pressing his argument that Obama was naive and inexperienced and doesn't understand a dangerous world.

"There are some advantages to experience and knowledge and judgment," he said. "And I honestly don't believe that Senator Obama has the knowledge or experience and has made the wrong judgments in a number of areas."

But Obama did not shrink from the foreign policy debate, arguing that on Iraq and Afghanistan, his judgment was superior to McCain's.

"Over the last eight years," he said, "this administration, along with Senator McCain, have been solely focused on Iraq. That has been their priority. That has been where all our resources have gone. In the meantime, bin Laden is still out there. He is not captured. He is not killed. Al-Qaida is resurgent."

Obama accused McCain of supporting Bush "90 percent of the time," including on an "orgy of spending" that had led to staggering federal deficits.

"To stand here after eight years and say you are going to lead on controlling spending ... is kind of hard to swallow," Obama said.

McCain, saying no one would elect him Miss Congeniality in the Senate, argued that "I have opposed the president on spending ... on torture of prisoners, on the way the Iraq war was conducted."

Throughout the debate, McCain brandished his credentials as someone who would attack spending. Obama said McCain's approach amounted to a hatchet when a scalpel was more appropriate.

Partisans on both sides saw bright spots and ultimate success for their candidate. Both accomplished much of what they hoped to do, without any serious mistakes.