WASHINGTON — Senator Obama said today that overall American interests have been hurt rather than helped by the Bush administration’s decision to increase troop strength in Iraq 18 months ago, and vowed to stick to his plan to withdraw combat troops within 16 months of becoming president.

Mr. Obama said his White House rival, Senator McCain, “has argued that the gains of the surge mean that I should change my commitment to end the war. But this argument misconstrues what is necessary to succeed in Iraq, and stubbornly ignores the facts of the broader strategic picture that we face.”

In a speech delivered in advance of an overseas trip, Mr. Obama said fighting al-Qaida and the Taliban in Afghanistan would be his top priority. Beyond that, he called for securing all nuclear weapons and materials from terrorists and rogue states, achieving energy security and rebuilding international alliances.

With his speech, Mr. Obama sought to cast the debate over the war in Iraq — and in particular the surge — in a wider context.

“Senator McCain wants to talk of our tactics in Iraq; I want to focus on a new strategy for Iraq and the wider world,” he said.

“This war distracts us from every threat that we face and so many opportunities we could seize. This war diminishes our security, our standing in the world, our military, our economy, and the resources that we need to confront the challenges of the 21st century. By any measure, our single-minded and open-ended focus on Iraq is not a sound strategy for keeping America safe.”

Mr. Obama’s successful run through the Democratic presidential primaries was fueled in part by his opposition to President Bush’s plans for an invasion of Iraq in 2003, and his 16-month timetable for a withdrawal has long been a staple of his speeches.

In recent weeks, though, Mr. McCain has challenged him to readjust his views to take the results of the so-called surge into account.

“Senator Obama is departing soon on a trip abroad that will include a fact-finding mission to Iraq and Afghanistan,” Mr. McCain said in prepared remarks.

“And I note that he is speaking today about his plans for Iraq and Afghanistan before he has even left, before he has talked to General Petraeus, before he has seen the progress in Iraq, and before he has set foot in Afghanistan for the first time. In my experience, fact-finding missions usually work best the other way around: First you assess the facts on the ground, then you present a new strategy.”

Mr. Obama answered bluntly.

“In the 18 months since the surge began, the strain on our military has increased, our troops and their families have borne an enormous burden, and American taxpayers have spent another $200 billion in Iraq,” he said.

He added that in Afghanistan, “June was our highest casualty month of the war. The Taliban has been on the offensive, even launching a brazen attack on one of our bases. Al-Qaida has a growing sanctuary in Pakistan.”

The speech billed as a major address by the campaign offered no new policy, but a high-profile explanation of his opposition to the war and his pledge to complete an American troop pullout within 16 months of becoming president. It also gave him a forum for criticizing Mr. Bush and Mr. McCain.

“I will end this war as president,” he said, speaking from a podium that said “Judgment to Lead.” Mr. Obama addressed the crowd with a line of American flags behind him.

Mr. Obama delayed his appearance for half an hour for a presidential news conference that the White House announced this morning during the same time that Obama was scheduled to be speaking just three blocks away.

President Bush was asked what advice he might give Mr. Obama as he prepared to visit Iraq. The president said he would ask Obama to listen carefully to General Petraeus, the top American commander, and the American ambassador, Ryan Crocker.

“It’s a temptation to let the politics at home get in the way, you know, with the considered judgment of the commanders,” Mr. Bush said. He defended his policy and maintained that the effort in Iraq was succeeding and acknowledged that the war in Afghanistan remained “a tough fight.”

Meanwhile, the New York Daily News reported that the Obama campaign altered its Web site to remove a statement that Mr. Bush’s surge of troops in Iraq “is not working.” Over the weekend, the site was changed to describe an “improved security situation” at the cost of American lives.

A campaign aide, Wendy Morigi, told the newspaper that Obama is “not softening his criticism of the surge. We regularly update the Web site to reflect changes in current events.”

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AP Special Correspondent David Espo contributed to this report.