Long before he won fame for his biographies of Douglas MacArthur, John F. Kennedy, and Winston Churchill, William Manchester was a young Marine corporal serving in the Pacific theater during World War II. He was severely wounded in the fighting on Okinawa, Japan, in June 1945, an experience he described in his wartime memoir, “Goodbye, Darkness.”

The violence on Okinawa was unimaginably savage. During the 12-week battle for the island, more than 12,500 Americans were killed and nearly 37,000 wounded. Japan’s losses were even more appalling. Determined to fight to the death rather than surrender the remote southern island, well over 100,000 Japanese soldiers were killed. The US Navy suffered the worst losses in its history. Waves of suicide attacks by Japanese aircraft resulted in the sinking of 34 American ships and the deaths of 5,000 US sailors. Bloody as the fighting was, however, everyone knew that it was only a foretaste of the losses Americans would sustain in a few months, when the climactic assault on Japan itself was expected to begin.

Which is why young Bill Manchester reacted with joyous relief when President Harry S. Truman decided to detonate two atomic bombs over Japan.

The carnage on Okinawa “reached its ultimate horrors in the last months of the war,” Manchester wrote. “You think of the lives which would have been lost in an invasion of Japan’s home islands — a staggering number of Americans but millions more of Japanese — and you thank God for the atomic bomb.”

At the time, Americans overwhelmingly agreed. But in the years that followed the bombing of Hiroshima on Aug. 6, 1945, and Nagasaki three days later, many critics condemned the use of the bomb as a great wrong. Noam Chomsky described the attacks as “among the most unspeakable crimes in history.” Historians Kai Bird and Martin Sherwin — whose Pulitzer Prize-winning “American Prometheus” is the basis for the new Hollywood biopic about J. Robert Oppenheimer, the so-called “father of the atomic bomb” — contend that the bombings were unnecessary since Japan was already effectively beaten. They quote Oppenheimer’s after-the-fact regret that Americans “used atomic weapons against an enemy which was essentially defeated.”

Considering the horrific toll inflicted by the bombs, it is entirely appropriate to debate the wisdom and morality of Truman’s decision. An estimated 200,000 Japanese died in the bombings and the firestorms and radiation poisoning they unleashed. It would be indecent not to ask whether there might have been a less ghastly way to bring the war to an end.

But I have always been persuaded that detonating the two atomic bombs was, as Secretary of War Henry Stimson put it, “our least abhorrent choice.”

While Japan in the summer of 1945 may have been “essentially defeated,” it was not ready to stop fighting. There’s evidence that Emperor Hirohito and some in his circle wanted to halt the bloodshed, but the military leaders who controlled the government saw things differently.

“They were determined to fight a final, all-out ‘decisive battle’ to bleed the United States invaders until the Americans sued for peace,” writes Evan Thomas in “Road to Surrender,” his new history of the atomic bombings. They were preparing to meet any Allied landing force with unprecedented, suicidal ferocity. “Had the United States been forced to invade, the bloodbath would have been unbearable to the US people, even before the besieged Japanese finally succumbed.”

It was only after the devastation of Hiroshima and Nagasaki that Japanese officials anxious to halt the killing finally prevailed. “We of the peace party were assisted by the atomic bomb in our endeavor to end the war,” said Koichi Kido, a top imperial adviser. The bomb, said Hisatsune Sakomizu, chief cabinet secretary in the Tokyo government, “was a golden opportunity given by heaven for Japan to end the war.”

Truman had other choices. He could have continued the incendiary firebombing of Japanese population centers, which had already incinerated a third of a million people in more than 65 cities — upward of 100,000 in Tokyo alone. He could have accelerated the destruction of Japanese food supplies, condemning millions to death by starvation. He could have allowed the Soviet Union, which declared war on Japan at the last minute, to invade the country from the north, slaughtering hundreds of thousands in the process.

In the end, he picked the option likely to cost the fewest human lives. Hiroshima and Nagasaki were horrifying, but they brought to an end the worst war in history. Awful as the bomb was, Truman was right to use it.