“I find wholly baffling the widespread belief today that the dropping of the Hiroshima and Nagasaki bombs was an immoral act, even possibly a war crime to rank with Nazi genocide.”

Report on the Potsdam Conference (1945)
Context: The world will note that the first atomic bomb was dropped on Hiroshima, a military base. That was because we wished in this first attack to avoid, insofar as possible, the killing of civilians. But that attack is only a warning of things to come. If Japan does not surrender, bombs will have to be dropped on her war industries and, unfortunately, thousands of civilian lives will be lost. I urge Japanese civilians to leave industrial cities immediately, and save themselves from destruction.

Announcing the Bombing of Hiroshima (1945)
Context: Sixteen hours ago an American airplane dropped one bomb on Hiroshima and destroyed its usefulness to the enemy. That bomb had more power than 20,000 tons of TNT. It had more than two thousand times the blast power of the British "Grand Slam" which is the largest bomb ever yet used in the history of warfare.
The Japanese began the war from the air at Pearl Harbor. They have been repaid many fold. And the end is not yet.

"President Truman Did Not Understand" http://www.peak.org/~danneng/decision/usnews.html in U.S. News & World Report (15 August 1960)
Variant: If the Germans had dropped atomic bombs on cities instead of us, we would have defined the dropping of atomic bombs on cities as a war crime, and we would have sentenced the Germans who were guilty of this crime to death at Nuremberg and hanged them.
As quoted in The Decision to Drop the Atomic Bomb (1996) by Dennis Wainstock, p. 122
Context: Suppose Germany had developed two bombs before we had any bombs. And suppose Germany had dropped one bomb, say, on Rochester and the other on Buffalo, and then having run out of bombs she would have lost the war. Can anyone doubt that we would then have defined the dropping of atomic bombs on cities as a war crime, and that we would have sentenced the Germans who were guilty of this crime to death at Nuremberg and hanged them?
But, again, don't misunderstand me. The only conclusion we can draw is that governments acting in a crisis are guided by questions of expediency, and moral considerations are given very little weight, and that America is no different from any other nation in this respect.

Quotes 1990s, 1995-1999, Z Magazine, July 1995

And the solution to the Iran crisis is..., JohannHari.com, January 22, 2006, 2007-01-26 http://www.johannhari.com/archive/article.php?id=783,

Statement of 1942, at the start of the bombing campaign against Germany, as quoted in "Sir Arthur Harris & The Lancaster Bomber" at The British Postal Museum and Archive http://postalheritage.org.uk/page/ww2stamps-bomberharris
Context: The Nazis entered this war under the rather childish delusion that they were going to bomb everyone else, and nobody was going to bomb them. At Rotterdam, London, Warsaw, and half a hundred other places, they put their rather naive theory into operation. They sowed the wind, and now they are going to reap the whirlwind.

Acceptance Speech, Army-Navy "Excellence" Award (16 November 1945)
Context: It is with appreciation and gratefulness that I accept from you this scroll for the Los Alamos Laboratory, and for the men and women whose work and whose hearts have made it. It is our hope that in years to come we may look at the scroll and all that it signifies, with pride. Today that pride must be tempered by a profound concern. If atomic bombs are to be added as new weapons to the arsenals of a warring world, or to the arsenals of the nations preparing for war, then the time will come when mankind will curse the names of Los Alamos and Hiroshima. The people of this world must unite or they will perish. This war that has ravaged so much of the earth, has written these words. The atomic bomb has spelled them out for all men to understand. Other men have spoken them in other times, and of other wars, of other weapons. They have not prevailed. There are some misled by a false sense of human history, who hold that they will not prevail today. It is not for us to believe that. By our minds we are committed, committed to a world united, before the common peril, in law and in humanity.

Remarks to the U.S. Congress (November 2017)

Howard Zinn on War (2000), Ch. 21: Just and Unjust War