“It is dreadful to be condemned to inactivity in this war which I prepared and initiated.”

Sometimes erroneously prepended to the opening line "The art of war is of vital importance to the State", but appears to be a variation of the Roman motto "Si vis pacem, para bellum". It's not clear who first misattributed this phrase to Sun Tzu. The earliest appearance of the phrase in Google Books is 1920, when it appeared in a pharmaceutical journal, but no attribution was given then.
Misattributed

Book 2, chapter 9, p. 271.
Compare Vegetius De Re Militari: "Qui desiderat pacem, praeparet bellum" (Let him who desires peace prepare for war).
Descriptio Cambriae (The Description of Wales) (1194)

Katniss Everdeen and Peeta Mellark, pp. 373-374 (closing words)
Source: The Hunger Games trilogy, The Hunger Games (2008)
Context: "One more time? For the audience?" he says. His voice wasn't angry. It's hollow, which is worse. Already the boy with the bread is slipping away from me.
I take his hand, holding on tightly, preparing for the cameras, and dreading the moment when I finally have to let go.

Speaking in Carnegie Hall, New York City, on 4 April 1919.

Context: As a soldier who has spent a quarter of his life in the study of the science of arms, let me tell you I went into the British Army believing that if you want peace you must prepare for war. I believe now that if you prepare thoroughly and efficiently for war, you get war.

Source: The Wisdom of W.E.B. Du Bois (2003), p. 75

On Protracted Warfare (1938)

Michael Lovett, in Ch. 18
Barbary Shore (1951)

Humanities interview (1996)
Context: I was very interested in the Great War, as it was called then, because it was the initial twentieth-century shock to European culture. By the time we got to the Second World War, everybody was more or less used to Europe being badly treated and people being killed in multitudes. The Great War introduced those themes to Western culture, and therefore it was an immense intellectual and cultural and social shock.
Robert Sherwood, who used to write speeches for Franklin D. Roosevelt, once noted that the cynicism about the Second War began before the firing of the first shot. By that time, we didn't need to be told by people like Remarque and Siegfried Sassoon how nasty war was. We knew that already, and we just had to pursue it in a sort of controlled despair. It didn't have the ironic shock value of the Great War.
And I chose to write about Britain because America was in that war a very, very little time compared to the British — just a few months, actually. The British were in it for four years, and it virtually destroyed British society.

War (1816)
Context: War is to be ranked among the most dreadful calamities which fall on a guilty world; and, what deserves consideration, it tends to multiply and perpetuate itself without end. It feeds and grows on the blood which it sheds. The passions, from which it springs, gain strength and fury from indulgence.