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

A wartime president honoured for peace, Obama became the first sitting US 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 overseas victory lap while thousands of US troops prepare to go off to war and millions of Americans remain jobless.

Just nine days after ordering 30,000 more US troops into battle in Afghanistan, 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.

But he also stressed the need to fight war according to “rules of conduct” that reject torture and other methods.

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

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

The list of Nobel peace laureates over the last 100 years includes transformative figures and giants of the world stage. They include  Mandela and others he has long admired, like George Marshall, who launched a postwar recovery plan for Europe.