President Barack Obama used his acceptance of the Nobel Peace Prize on Thursday to defend the idea that some wars were necessary and just, to remind the world of the burden the United States had borne in the fight against oppression and to appeal for greater international efforts for peace. 'We must begin by acknowledging the hard truth: We will not eradicate violent conflicts in our lifetimes,' Obama said, addressing the paradox of receiving an award for peace as commander in chief of a nation that is escalating the war in Afghanistan as it continues to fight in Iraq. 'There will be times when nations -- acting individually or in concert -- will find the use of force not only necessary but morally justified.' He delivered a mix of realism and idealism, implicitly criticizing both the Rev. Dr. Martin Luther King Jr. as inadequately appreciating the dangers of the world and former President George W. Bush as too quick to set aside fundamental American values in pursuit of security. And he embraced the concept of American exceptionalism, the idea that the United States has a special role as a defender of liberty, even as he promoted multilateralism. In that way, he continued a pattern evident throughout his public career of favoring pragmatism over absolutes. The address -- delivered at once to a European audience that has grown skeptical about U.S. power and to a domestic audience watching closely to see how he would handle the acceptance of an award that even he acknowledged he did not yet deserve -- represented one of the broadest declarations of his foreign policy doctrine. He said that others deserved the award more, noting that his 'accomplishments are slight,' but he accepted the prize with a strong endorsement of America's place in the world. 'Whatever mistakes we have made, the plain fact is this,' Obama said. 'The United States of America has helped underwrite global security for more than six decades with the blood of our citizens and the strength of our arms.' The Nobel lecture, a 36-minute address that the president and his aides completed on an overnight flight from Washington, carried echoes of several American presidents, from Jimmy Carter to Bush, but he singled out one above all: John F. Kennedy. Obama cited Kennedy's focus on 'not a sudden revolution in human nature but on a gradual evolution in human institutions.' He called for more robust international sanctions against nations like Iran and North Korea that defy demands for them to curtail their nuclear programs. Weeks after being criticized for not speaking out more publicly in defense of human rights while in China, he suggested that quiet diplomacy was sometimes the most productive path, even if it 'lacks the satisfying purity of indignation.' The ceremony was the focal point of a series of events celebrating Obama's entry into the ranks of Nobel laureates. On Thursday night, the president and his wife, Michelle, appeared in a window of the Grand Hotel, waving to thousands of people below who had gathered for a torch-light parade. Trumpets sounded when Obama walked down the long aisle of a soaring auditorium to deliver his speech. The president escorted his wife, who took her seat in the front row, before he assumed his position on the stage and faced the king and queen of Norway. The Nobel chairman, Thorbjorn Jagland, opened the ceremony by explaining how the committee came to its decision two months ago. He said Obama's leadership had been a 'call to action for all of us.' As he invoked the story of King, the winner of the prize in 1964, he turned to Obama, saying, 'Dr. King's dream has come true.' Obama pursed his lips and nodded gently as the audience applauded loudly. When he was presented his gold medal and Nobel diploma, he received a standing ovation that stretched for more than a minute. The crowd did not rise again until the conclusion of his remarks. Obama's speech was sober, with his remarks only sparingly interrupted by applause. He was applauded when he renewed his pledge to ban torture and close the prison at the U.S. base at Guantanamo Bay, Cuba. 'We lose ourselves when we compromise the very ideals that we fight to defend,' Obama said. 'And we honor those ideals by upholding them not when it is easy, but when it is hard.' To a European audience of academics, diplomats and Nobel laureates, Obama said that there was 'a deep ambivalence about military action today,' which he said he suspected was rooted in 'a reflexive suspicion of America.' But he offered a forceful defense of the United States, saying the lessons of history should ease those suspicions. The president did not dwell on the specifics of his announcement last week that he would send 30,000 more U.S. troops to Afghanistan. But that decision, which attracted scores of peaceful demonstrators here, set the framework that Obama returned to again and again as he sought to explain his policy as an extension of the post-World War II system that contained the cold war. 'A decade into a new century, this old architecture is buckling under the weight of new threats,' Obama said. 'The world may no longer shudder at the prospect of war between two nuclear superpowers, but proliferation may increase the risk of catastrophe. Terrorism has long been a tactic, but modern technology allows a few small men with outsize rage to murder innocents on a horrific scale.' Obama, who is scheduled to stay in Oslo for about 26 hours, had miffed some Norwegians by not participating in some of the traditional events surrounding the peace prize ceremony, including a luncheon and a concert on Friday evening. Obama, sensitive to the criticism, explained the brevity of his visit. 'I only wish that my family could stay longer in this wonderful country,' he told reporters, 'but I still have a lot of work to do back in Washington, D.C., before the year is done.' The president is scheduled to return to Washington on Friday. 