HOUSTON, March 6 ? Had he been sitting in the Oval Office last weekend as rebel forces were threatening to enter Port-au-Prince, Senator John Kerry says, he would have sent an international force to protect Haiti’s widely disliked elected leader, Jean-Bertrand Aristide.

“I would have been prepared to send troops immediately, period,” Mr. Kerry said on Friday, expressing astonishment that President Bush, who talks of supporting democratically elected leaders, withheld any aid and then helped spirit Mr. Aristide into exile after saying the United States could not protect him.

“Look, Aristide was no picnic, and did a lot of things wrong,” Mr. Kerry said. But Washington “had understandings in the region about the right of a democratic regime to ask for help. And we contravened all of that. I think it’s a terrible message to the region, democracies, and it’s shortsighted.”

Mr. Kerry’s critique on Haiti, which Bush campaign aides dismissed as political, was emblematic of how he is already using foreign policy and national security issues in his contest with the president.

In his first in-depth interview on foreign affairs since effectively winning the Democratic nomination, Mr. Kerry hop-scotched around the world in the course of an hour. He took issue with Mr. Bush’s judgment beyond their well-aired differences on Iraq, questioning his handling of North Korea, the Mideast peace process and the spread of nuclear weapons and arguing that he would rewrite the Bush strategy that makes pre-emption a declared, central tenet of American policy.

Mr. Kerry is trying a bit of election-season pre-emption of his own, attempting to short-circuit the White House argument that he is too much of a straddler, too indecisive and too captivated by the nuances of foreign policy to defend American interests.

“People will know I’m tough and I’m prepared to do what is necessary to defend the United States of America, and that includes the unilateral deployment of troops if necessary,” said Mr. Kerry, who has rarely used the word “unilateral” in the campaign except to describe how Mr. Bush has alienated allies. “But my standard is very different from George Bush’s.”

But so far, Mr. Kerry has not described that standard in detail. In the interview on his plane, Mr. Kerry said he was reluctant to define how he would act in specific situations ? particularly in murky cases like Pakistan ? because conditions could change by next year.

Yet, signaling how he plans to use the questionable intelligence about Iraq to chip away Mr. Bush’s credibility, he added that if he committed troops to battle, he would do it with “full disclosure and full vetting of the intelligence to the American people.”

But the core of Mr. Kerry’s argument in the interview was that divisions within Mr. Bush’s foreign policy team have frozen the art of preventative diplomacy and kept Secretary of State Colin L. Powell from doing his job.

“I think simply Powell, who I know, like and admire, has been never permitted to be fully a secretary of state in the way that I envision the secretary of state,” he said, describing how he believes that Mr. Powell has been regularly undercut by the administration’s more hawkish members, led by Vice President Dick Cheney. “I think Powell ? I’m not sure they didn’t lock the keys to the airplane up sometimes.”

Mr. Kerry warms to these topics in a way he never quite seems to do when talking about his multistep plans to reform health care or roll back parts of Mr. Bush’s tax cuts.

Years on the Senate Foreign Relations committee have given him a fluidity in talking about the world’s hot spots. On Friday he spoke of his long tutorials about China with Lee Kwan Yew, the authoritarian founder of modern Singapore and now its senior minister. He slipped in a reference to his conversations with world leaders at the annual World Economic Forum in Davos, Switzerland, an exclusive event where he used to be a regular attendee.

He said he might take a foreign trip during the campaign ? he would not say where ? an event that would emphasize a diplomatic style distinct from Mr. Bush’s.

Yet some members of his growing foreign policy team ? some days it seems as if every former Clinton administration aide is now scrambling to climb aboard, forgetting their brief dalliances with Mr. Kerry’s former primary rivals ? worry that such talk could backfire on the campaign trail. Mr. Kerry, they concede, still has a hard time mustering the clear, declarative sentences and bedrock precepts that have become Mr. Bush’s trademark.

“It’s hard to imagine John banging his fist and declaring that countries are `either with us or against us,’ said one adviser who speaks frequently with the Massachusetts senator. “Is that good or bad in the general election? We don’t know.”

Moreover, Mr. Kerry clearly has not yet mastered the Clintonian knack of engagingly breaking down complex problems into simple, accessible terms, no matter his audience. On the campaign trail he regularly indicts the administration’s “arrogant, inept, reckless, ideological” foreign policy and warns that “even the United States of America needs a few friends on this planet.”

But when pressed for details, Mr. Kerry can veer into the dry, Latin-heavy jargon of policy journals.

He is trying, though, to speak in far more concrete terms. In a radio address broadcast Saturday morning, a response to Mr. Bush’s weekly radio commentary, Mr. Kerry dwelled at length on the stories of American military units scrounging for private donations of steel plates for their Humvees and body armor before they deploy to Iraq. “This administration has given billions to Halliburton and requested $82 million to protect Iraq’s 36 miles of coastline,” he said. “But they call this basic body armor a `nonpriority’ item.”

In turn, the Bush campaign is scouring Mr. Kerry’s voting record for evidence of weapons systems he has sought to kill, and it is whispering to reporters about the contrast between Mr. Kerry’s desire to shrink the intelligence services after the cold war and his declarations today that they are unprepared for the task of combating terrorism.

But to talk to Mr. Bush’s advisers and then to Mr. Kerry is to hear a very different description of America’s influence in the world. For example, Condoleezza Rice, the national security adviser, argued recently that the decision to confront Saddam Hussein sent such a strong message worldwide that Col. Muammar el-Qaddafi, the Libyan leader, agreed to surrender his nuclear and chemical weapons. In turn, she contended, that helped unwind the nuclear proliferation network built by the Pakistani scientist Abdul Qadeer Khan.

Mr. Kerry waved that argument away: “Everybody who knows anything about Libya and Qaddafi knows that is not what brought him around. They will claim it, they want the linkage, but it is not true, like much of what they say.” He added, “that deal was on the table several years ago” as Libya sought to settle the Pan Am Flight 103 terrorism case.

“No matter how much they bluster and futz, they can’t fake it,” he said.

But in several cases Mr. Kerry declined to say how he would handle some of the stickiest issues: whether to reward Pakistan for its aid against Al Qaeda, for example, or punish it for failing to crack down on what was clearly one of the world’s most dangerous nuclear proliferation networks, based in its own laboratories.

He said, however, that he had no confidence in White House assurances that Pakistan’s nuclear weapons were safe from terrorists.

Steve Schmidt, a spokesman for the Bush campaign, dismissed Mr. Kerry’s remarks. “Senator Kerry has a record of changing his positions when he feels political pressure,” Mr. Schmidt said Saturday. “When his support for the war in Iraq became politically unpopular in the Democratic primary he changed his position, declared himself an antiwar candidate, and voted against body armor and other important equipment for troops in the field.”

In his conversations on foreign policy, sooner or later Mr. Kerry returns to the touchstone of his early adult life, Vietnam. He compared the Bush administration’s participation in the exile of Mr. Aristide last weekend to the coup four decades ago that ousted another unpopular authoritarian leader: President Ngo Dinh Diem of South Vietnam. Mr. Bush’s more hawkish aides, he argued, have failed to learn how the efforts to change a region’s dynamics by changing its government almost always backfire.

In Haiti’s case, he contended that if he had been in Mr. Bush’s shoes, “I would not have allowed it to arrive at where it was,” with mobs roaming the streets of Haiti’s cities.

Mr. Bush’s aides, led by Mr. Powell, said last week that such critiques distort of the administration’s efforts. The crisis grew from Mr. Aristide’s own actions and his sponsorship of the marauding gangs, Mr. Powell said last week, and the United States decided not to prop him up after he had lost his legitimacy.

Mr. Kerry charges that a similar lack of constant attention led the administration to avoid dealing with the North Korean crisis for the first 18 months of Mr. Bush’s presidency and that even now, Mr. Bush is unwilling to engage in serious negotiations. It was an example, he said, of the president’s dealing first with the less threatening problem, Iraq, because it was the easier to solve.

“There’s a reason the Bush administration walked that backwards and chose Iraq,” he said. “And the reason is in the first eight hours of a conflict with North Korea, you’d have over a million casualties, and they knew that in Iraq you wouldn’t.”