It was no surprise that Iraq dominated the first debate between President Bush and Democratic challenger John F. Kerry on Thursday night, but rarely have the differences between the two men -- and the choices for the country -- been stated so clearly and with such passion.

Bush and Kerry differed on almost every aspect of the war in Iraq and on other major foreign policy issues such as North Korea and Iran. They disagreed over whether former president Saddam Hussein posed a serious threat to the United States at the time Bush took the country to war there. They disagreed on whether it was right to go to war as Bush did. They differed on whether the president has a plan to secure the peace. And they parted company on whether the certitude Bush has displayed as president has advanced U.S. security or weakened it.

If Republicans had hoped Bush could put Kerry away with a strong performance on terrain that has been his strongest suit, they are likely to be disappointed, as the Democrat constantly challenged the president to answer for his policies. Both men accomplished many of the goals their advisers had set out in the days before the debate and probably reinforced the strong backing each already has among his most committed supporters. But for those voters who remain undecided, Bush and Kerry may have only whetted appetites for their two remaining debates.

This was a debate shorn of gimmicks, gaffes, canned one-liners, gotcha moments or even many light-hearted asides. It was as serious as the times in which this campaign is being waged. Bush and Kerry gave as good as they got and laid out for the country a choice between Bush's determination to stay on the course he has been following in Iraq or what Kerry said would be a genuine change in the direction of policy there.

Bush appeared defensive at the start of the 90-minute debate, and at times the camera caught him scowling or frowning as Kerry relentlessly attacked his record on Iraq. But as the debate continued, he made a passionate defense of the values that are at the foundation of his foreign policy: taking the fight to terrorists and spreading freedom across the planet.

Kerry, who was under great pressure to perform well, repeatedly presented his case that the president has led the country astray and that only a change in leadership can alter the equation in Iraq and attract the support of other countries in sharing more of the burden. He also sought to answer doubts about himself by trying to show that he would be resolute in fighting terrorists, albeit in a different way.

Instant polls judged Kerry the clear winner, but Kerry came into the debate knowing he had to begin to undo the damage the Bush campaign has inflicted on him and reverse public perceptions that Bush is better equipped to deal with Iraq and to fight terrorists -- and that the president is far more likable personally.

Whether he began to reverse those perceptions will not be clear immediately. His demeanor may have helped to counter the image Bush's ads have tried to create, but he spent little time explaining apparent contradictions in his positions on Iraq and may have more work to do on that front in the next two debates.

For 90 minutes, Bush and Kerry stood opposite each other on the campus of the University of Miami and described strikingly different approaches to the world and sharply contrasting portraits of each other's character. For the two men, foreign policy was defined almost exclusively through the prism of Iraq, with no discussion of the conflict between the Israelis and the Palestinians, Middle East oil, Saudi Arabia or Pakistan. Bush repeatedly tried to undermine Kerry as someone who has shifted with the political winds on Iraq. Kerry challenged Bush's record in Iraq as indefensible.

Kerry faced the more daunting challenge going into Thursday's debate, given the state of the presidential race that has shown Bush in the lead, and his advisers were looking for the kind of performance that would begin to change the dynamic of the campaign heading into the next debate, on Oct. 8. But Bush also came to the debate with obstacles, principally the mounting bloodshed in Iraq and the strength of the insurgency there, and he needed to keep Kerry on the defensive as much as possible.

Bush portrayed Kerry as a politician without core convictions who has repeatedly changed his mind and said that by describing Iraq as "the wrong war" had called into question his ability to lead U.S. troops or rally the world. Kerry described Bush as unwilling to acknowledge the realities of an Iraq that experienced another bloody day Thursday and who has made a series of wrong decisions, from rushing to war to failing to develop a plan for peace to shattering relations with U.S. allies.

When PBS's Jim Lehrer, the moderator, asked Bush toward the end of the debate whether there were any character flaws in Kerry that would disqualify the challenger from serving as commander in chief, Bush said: "He changes positions on something as fundamental as what you believe in your core, in your heart of hearts, is right in Iraq. You cannot lead if you send mixed messages. Mixed messages send the wrong signals to our troops. Mixed messages send the wrong signals to our allies. Mixed messages send the wrong signals to the Iraqi citizens."

When he got his turn to respond, Kerry said, "I'm not going to talk about a difference of character. I don't think that's my job or my business. But let me talk about something that the president just sort of finished up with. Maybe someone would call it a character trait, maybe somebody wouldn't. But this issue of certainty. It's one thing to be certain, but you can be certain and be wrong."

"I have a difference with this president," he said. "I believe when we're strongest when we reach out and lead the world and build strong alliances. I have a plan for Iraq. I believe we can be successful. I'm not talking about leaving. I'm talking about winning. And we need a fresh start, a new credibility, a president who can bring allies to our side."

But when Kerry said he doubts that Bush really sees the deterioration in Iraq, the president challenged Kerry's ability to find a strategy that would work, given his stances on the war.

"I don't see how you can lead this country to succeed in Iraq if you say wrong war, wrong time, wrong place," he said. "What message does that send our troops? What message does that send to our allies? What message does that send the Iraqis? No, the way to win this is to be steadfast and resolved and to follow through on the plan that I've just outlined."

Bush has maintained a small but steady lead over Kerry in the national polls since the Republican National Convention in New York. In addition, Kerry's campaign has gradually withdrawn from a number of states they had targeted as potential pickups, narrowing his electoral options as the campaign heads toward its final month.

Kerry partisans said immediately after the debate that they were pleased with his performance, while Bush backers said they were confident that as long as the campaign remains focused on foreign policy, the debate will benefit the president.

Few strategists believed that this first of three presidential debates will, by itself, fundamentally change the shape of the campaign, but for voters wondering whether the choice is as stark as the two sides had been portraying it, the session provided a resounding answer. It sets the stage for what promises to be a ferocious campaign over the next 41/2 weeks.

Both men had something to prove before a national audience, and instant polls judged Sen. John F. Kerry the victor.