OSLO (AP) — President Barack Obama evoked the cause of a ‘just war’ on Thursday, accepting his Nobel Peace Prize just nine days after sending 30,000 more U.S. troops to battle in Afghanistan but promising to use the prestigious prize to “reach for the world that ought to be.”

Obama became the first sitting U.S. president in 90 years and the third ever to win the prize — some say prematurely. He and his wife, Michelle, whirled through a day filled with Nobel pomp and ceremony in this Nordic capital.

Obama delivered a Nobel acceptance speech that he saw as a treatise on war’s use and prevention. He crafted much of the address himself and the scholarly remarks — at about 4,000 words — were nearly twice as long as his inaugural address.

WATCH PRESIDENT OBAMA ACCEPT THE NOBEL PEACE PRIZE

“I face the world as it is,” Obama said, refusing to renounce war for his nation or under his leadership, saying that he is obliged to protect and defend the United States.

“A nonviolent movement could not have halted Hitler’s armies. Negotiations cannot convince al-Qaida’s leaders to lay down their arms,” Obama said. “To say that force is sometimes necessary is not a call to cynicism, it is a recognition of history.”

The president laid out the circumstances where war is justified — in self-defense, to come to the aid of an invaded nation, or on humanitarian grounds, such as when civilians are slaughtered by their own government or a civil war threatens to engulf an entire region.

“The belief that peace is desirable is rarely enough to achieve it,” he said.

He also spoke bluntly of the cost of war, saying of the Afghanistan buildup he just ordered that “some will kill, some will be killed.”

“No matter how justified, war promises human tragedy,” he said.

Obama also emphasized alternatives to violence, stressing the importance of both diplomatic efforts and tough sanctions to confront nations such as Iran or North Korea, which defy international demands to halt their nuclear programs, or those such as Sudan, Congo or Burma that brutalize their citizens.

“Let us reach for the world that ought to be,” Obama said. “We can understand that there will be war, and still strive for peace.”

Obama was staying in Oslo only about 24 hours and skipping the traditional second day of festivities. This miffed some in Norway but reflects a White House that sees little value in extra pictures of the president, his poll numbers dropping at home, taking an overseas victory lap while thousands of U.S. troops prepare to go off to war and millions of Americans remain jobless.

In awarding the prize to Obama, the Nobel panel cited his call for a world free of nuclear weapons, for a more engaged U.S. role in combating global warming, for his support of the United Nations and multilateral diplomacy and for broadly capturing the attention of the world and giving its people “hope.”

But the Nobel committee made its announcement in October when he wasn’t even nine months on the job, recognizing that his aspirations more than his achievements.

Echoing the surprise that greeted his win, Obama started his 36-minute speech by saying that others who have done more and suffered more may better deserve the honor.

“I am at the beginning, and not the end, of my labors on the world stage,” the president said. “Compared to some of the giants of history who have received this prize … my accomplishments are slight.”

The list of Nobel peace laureates over the last 100 years includes transformative figures and giants of the world stage. They include heroes of the president, such as the Rev. Martin Luther King Jr., Nelson Mandela and others he has long admired, like George Marshall, who launched a postwar recovery plan for Europe.

Earlier, Obama had said that the criticism might recede if he advances some of his goals. But, he added, proving doubters wrong is “not really my concern.”

“If I’m not successful, then all the praise in the world won’t disguise that fact,” he said.

The timing of the award ceremonies, coming so soon after Obama’s Afghanistan announcement, lent inspiration to peace activists.

The president’s motorcade arrived at Oslo’s high-rise government complex for Obama’s meeting with Norwegian Prime Minister Jens Stoltenberg as a few dozen anti-war protesters gathered behind wire fences nearby. Dressed in black hoods and waving banners, the demonstrators banged drums and chanted anti-war slogans.

“The Afghan people are paying the price,” some shouted.

Greenpeace and anti-war activists planned larger demonstrations later. Protesters have plastered posters around the city, featuring an Obama campaign poster altered with skepticism to say, “Change?”

Other observers defended the decision to give Obama the prize.

“It’s a special situation. He is the U.S. president and he can’t be a pacifist, if we are going to be realistic,” said Hans-Martin Flaaten, a 36-year-old art historian. “I believe peace is what he stands for and he defended that principle very well.”

Flaaten was among some 500 people who watched the Nobel Ceremony on a large outdoor screen set up on fortress rampants near City Hall.

The debate at home over his Afghanistan decision also followed the president here. Obama told reporters that the July 2011 date he set for the U.S. withdrawal from Afghanistan to begin will not slip — but that the pace of the full withdrawal will be gradual and condition-based.

“We’re not going to see some sharp cliff, some precipitous drawdown,” Obama said.

Obama’s first stop in Oslo was the Norwegian Nobel Institute, where the Nobel committee meets to make its decisions. After signing the guest book, Obama told reporters he thanked the committee and noted the pictures of former winners filling the wall, many of whom gave “voice to the voiceless.”

In the evening, Obama is expected to wave to a torchlight procession from his hotel balcony and stroll with Norwegian royalty to a dinner banquet. He will offer comments a second time there and cap his brisk jaunt to Europe.

The president and his wife, Michelle, arrived here in the morning, coming off Air Force One holding hands and smiling. Obama was due back in Washington by midday Friday.

The Nobel honor comes with a $1.4 million prize. The White House says Obama will give that to charities but has not yet decided which ones.

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Associated Press writers Matti Huuhtanen and Ian MacDougall contributed to this report.

Copyright 2009 The Associated Press.