“The Essence of War is Violence. Moderation in War is Imbecility.”

Must We Go to War? (1937)

Source: The Ways and Power of Love (1954), p. xi

Variant translation: To introduce into the philosophy of war a principle of moderation would be an absurdity.
As quoted in The Campaign of 1914 in France and Belgium‎ (1915) by George Herbert Perris, p. 56.
Source: On War (1832), Book 1, Chapter 1, Section 3, Paragraph 3

A Prescription for Hope (1985)
Context: The hope of a benevolent civilization was shattered in the blood-soaked trenches of the First World War. The "war to end all wars" claimed sixteen million lives, and left embers which kindled an even more catastrophic conflagration.
Over the sorry course of 5,000 years of endless conflicts, some limits had been set on human savagery. Moral safeguards proscribed killing unarmed civilians and health workers, poisoning drinking waters, spreading infection among children and the disabled, and burning defenseless cities. But the Second World War introduced total war, unprincipled in method, unlimited in violence, and indiscriminate in victims. The ovens of Auschwitz and the atomic incineration of Hiroshima and Nagasaki inscribed a still darker chapter in the chronicle of human brutality. The prolonged agony which left 50 million dead did not provide an enduring basis for an armistice to barbarism. On the contrary, arsenals soon burgeoned with genocidal weapons equivalent to many thousands of World War II's.
The advent of the nuclear age posed an unprecedented question: not whether war would exact yet more lives but whether war would preclude human existence altogether.

Source: Kafka on the Shore

Variant translation: War is an act of violence which in its application knows no bonds.
As quoted in The Campaign of 1914 in France and Belgium‎ (1915) by George Herbert Perris, p. 56.
Source: On War (1832), Book 1, Chapter 1, Section 3, Paragraph 8

"Why Are We in Kosovo?", The New York Times (2 May 1999)
Context: Not surprisingly, the Serbs are presenting themselves as the victims. (Clinton equals Hitler, etc.) But it is grotesque to equate the casualties inflicted by the NATO bombing with the mayhem inflicted on hundreds of thousands of people in the last eight years by the Serb programs of ethnic cleansing.
Not all violence is equally reprehensible; not all wars are equally unjust.
No forceful response to the violence of a state against peoples who are nominally its own citizens? (Which is what most "wars" are today. Not wars between states.) The principal instances of mass violence in the world today are those committed by governments within their own legally recognized borders. Can we really say there is no response to this?

1990s, Nobel Prize acceptance speech (1993)

Source: On War (1832), Book 1, Chapter 1, paragraph 2.