OSLO: President Barack Obama entered the pantheon of Nobel Peace Prize winners with humble words Thursday, acknowledging his own few accomplishments while delivering a robust defense of war and promising to use the prestigious prize to "reach for the world that ought to be."

A wartime president honored for peace, Obama became the first sitting U.S. president in 90 years and the third ever to win the prize — some say prematurely. In this damp, chilly Nordic capital to pick it up, he and his wife, Michelle, whirled through a day filled with Nobel pomp and ceremony.

And yet Obama was staying here 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 overseasvictory lap while thousands of U.S. troops prepare to go off to war and millions of Americans remain jobless.

Just nine days after ordering 30,000 more U.S. troops into battle inAfghanistan, 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.

In them, Obama refused to renounce war for his nation or under his leadership, saying defiantly that "I face the world as it is" and 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 and 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.

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