“Finally, there is a compelling moral argument against war in Iraq. Military force is justified only in self-defense; naked aggression is the province of dictators and rogue states. This is the danger of a new "preemptive first strike" doctrine. America is the most moral nation on earth, founded on moral principles, and we must apply moral principles when deciding to use military force.”

2009, Nobel Prize acceptance speech (December 2009)
Context: In today's wars, many more civilians are killed than soldiers; the seeds of future conflict are sown, economies are wrecked, civil societies torn asunder, refugees amassed, 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: 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 Jr. 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 work, I am living testimony to the moral force of non-violence. I know there's nothing weak — nothing passive — nothing naïve — in the creed and lives of Gandhi and King.
But as a head of state sworn to protect and defend my nation, I cannot be guided by their examples alone. I face the world as it is, and cannot stand idle in the face of threats to the American people. For make no mistake: Evil does exist in the world. A non-violent movement could not have halted Hitler's armies. Negotiations cannot convince al Qaeda's leaders to lay down their arms. To say that force may sometimes be necessary is not a call to cynicism — it is a recognition of history; the imperfections of man and the limits of reason.

Liberty vs Socialism
Context: There is no moral argument that justifies using the coercive powers of government to force one person to bear the expense of taking care of another. If that person is too resolute in his refusal to do so, what is the case for imposing fines, imprisonment or death? You say, "Death! Aren't you exaggerating, Williams?" Say he tells the agents of Congress that he'll pay his share of the constitutionally mandated functions of government but refuse to pay the health costs of a sick obese person or a cyclist who becomes a vegetable, what do you think the likely course of events will be? First, he'd be threatened with fines, imprisonment or property confiscation. Refusal to give in to these government sanctions would ultimately lead to his being shot by the agents of Congress.

Some Reflections on Peace in Our Time (1950)

Speech at the National Press Club (2004)

"Morality and Revolution"
The Road to Revolution (2008)

H.L.A. Hart, The Concept of Law, 1961. p. Xviii; introduction.

Quoted in "Behind the Japanese Mask" - Page 43 - by Jesse Frederick Steiner - History - 1943

Source: The Sword or the Cross, Which Should be the Weapon of the Christian Militant? (1921), Ch.4 p. 62-63

Source: An American Dilemma (1944), p. 1021