But he also praised the peacemakers of the past and said the world can and should still strive for peace.

The following is a transcript of Obama's acceptance speech:

Your majesties, your royal highnesses, distinguished members of the Norwegian Nobel Committee, citizens of America and citizens of the world:

I receive this honor with deep gratitude and great humility. It is an award that speaks to our highest aspirations -- that for all the cruelty and hardship of our world, we are not mere prisoners of fate. Our actions matter, and can bend history in the direction of justice.

And yet I would be remiss if I did not acknowledge the considerable controversy that your generous decision has generated. In part, this is because I am at the beginning, and not the end, of my labors on the world stage. Compared to some of the giants of history who have received this prize -- Schweitzer and King; Marshall and Mandela -- my accomplishments are slight. And then there are the men and women around the world who have been jailed and beaten in the pursuit of justice; those who toil in humanitarian organizations to relieve suffering; the unrecognized millions whose quiet acts of courage and compassion inspire even the most hardened of cynics. I cannot argue with those who find these men and women -- some known, some obscure to all but those they help -- to be far more deserving of this honor than I.

But perhaps the most profound issue surrounding my receipt of this prize is the fact that I am the commander in chief of a nation in the midst of two wars. One of these wars is winding down. The other is a conflict that America did not seek; one in which we are joined by forty-three other countries -- including Norway -- in an effort to defend ourselves and all nations from further attacks.

Still, we are at war, and I am responsible for the deployment of thousands of young Americans to battle in a distant land. Some will kill. Some will be killed. And so I come here with an acute sense of the cost of armed conflict -- filled with difficult questions about the relationship between war and peace, and our effort to replace one with the other.

These questions are not new. War, in one form or another, appeared with the first man. At the dawn of history, its morality was not questioned; it was simply a fact, like drought or disease -- the manner in which tribes and then civilizations sought power and settled their differences.

Over time, as codes of law sought to control violence within groups, so did philosophers, clerics and statesmen seek to regulate the destructive power of war. The concept of a "just war" emerged, suggesting that war is justified only when it meets certain preconditions: if it is waged as a last resort or in self-defense; if the forced used is proportional, and if, whenever possible, civilians are spared from violence.

For most of history, this concept of just war was rarely observed. The capacity of human beings to think up new ways to kill one another proved inexhaustible, as did our capacity to exempt from mercy those who look different or pray to a different God. Wars between armies gave way to wars between nations -- total wars in which the distinction between combatant and civilian became blurred. In the span of 30 years, such carnage would twice engulf this continent. And while it is hard to conceive of a cause more just than the defeat of the Third Reich and the Axis powers, World War II was a conflict in which the total number of civilians who died exceeded the number of soldiers who perished.

In the wake of such destruction, and with the advent of the nuclear age, it became clear to victor and vanquished alike that the world needed institutions to prevent another World War. And so, a quarter century after the United States Senate rejected the League of Nations -- an idea for which Woodrow Wilson received this prize -- America led the world in constructing an architecture to keep the peace: a Marshall Plan and a United Nations, mechanisms to govern the waging of war, treaties to protect human rights, prevent genocide and restrict the most dangerous weapons.

In many ways, these efforts succeeded. Yes, terrible wars have been fought, and atrocities committed. But there has been no Third World War. The Cold War ended with jubilant crowds dismantling a wall. Commerce has stitched much of the world together. Billions have been lifted from poverty. The ideals of liberty, self-determination, equality and the rule of law have haltingly advanced. We are the heirs of the fortitude and foresight of generations past, and it is a legacy for which my own country is rightfully proud.

A decade into a new century, this old architecture is buckling under the weight of new threats. 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 outsized rage to murder innocents on a horrific scale.

Moreover, wars between nations have increasingly given way to wars within nations. The resurgence of ethnic or sectarian conflicts; the growth of secessionist movements, insurgencies, and failed states; have increasingly trapped civilians in unending chaos. In today's wars, many more civilians are killed than soldiers; the seeds of future conflict are sewn, economies are wrecked, civil societies torn asunder, refugees amassed and children scarred.

I do not bring with me today a definitive solution to the problems of war. What I do know is that meeting these challenges will require the same vision, hard work and persistence of those men and women who acted so boldly decades ago. And it will require us to think in new ways about the notions of just war and the imperatives of a just peace.

We must begin by acknowledging the hard truth that we will not eradicate violent conflict in our lifetimes. There will be times when nations -- acting individually or in concert -- will find the use of force not only necessary but morally justified.

I make this statement mindful of what Martin Luther King said in this same ceremony years ago -- "Violence never brings permanent peace. It solves no social problem: it merely creates new and more complicated ones." As someone who stands here as a direct consequence of Dr. King's life's work, I am living testimony to the moral force of nonviolence. I know there is nothing weak -- nothing passive -- nothing naive -- in the creed and lives of Gandhi and King.

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