We have been hearing a lot about the 70th anniversary of the first use of a nuclear weapon on human beings, in Hiroshima on Aug. 6, 1945. The more important anniversary, however, is Aug. 9, when the last nuclear weapon was used in war, on the city of Nagasaki.

It was predictable that atomic bombs would be used as soon as they were developed in 1945. It was the sixth year of World War II, and more than 60 million people had been killed already. But nobody would have believed then that nuclear weapons would not be used again in future wars.

We cannot be sure that they never will be used in war again, of course, but 70 years is already an impressive accomplishment. How did we manage that? One way to answer that question is to consider the behavior of U.S. President Harry S. Truman, who was the man who decided to drop the first atomic bombs in 1945 — and the first man to decide not to drop them, in 1951.

Truman’s decision to drop the bombs in 1945 probably didn’t seem as momentous to him at the time as it looks now. Killing tens of thousands of civilians in cities by mass bombing (Hamburg, Dresden, Tokyo) was practically routine by 1945, and the atomic bombs would have seemed like just a more efficient way of doing the same thing.

Besides, the fact that Japanese cities could now be destroyed by a single plane carrying a single bomb might well shock the Japanese government into surrendering. That would spare the lives of all the American soldiers (an estimated 46,000) who would die if Japan had to be invaded.

Truman had fought in World War I (he was the only major Allied war leader who did). Although he was not generally seen as an imaginative man, he would have been vividly aware of the ordeal that awaited American soldiers if they had to invade Japan. He would also have been conscious that the U.S. public would never forgive him if they found out that he had the bomb but didn’t use it to save those soldiers’ lives.

So he gave the orders and the bombs fell, adding a last quarter-million lives to that 60 million death toll. But 5½ years later, when U.S. forces in Korea were fleeing south after Chinese troops intervened in the war there (“the big bug-out”), Truman behaved quite differently.

It may or may not be true that U.S. Gen. Douglas MacArthur, who commanded the United Nations troops in Korea (including a third of a million Americans), wanted to drop atomic bombs on China’s Manchurian provinces to cut the supply lines of the Chinese troops in Korea. It is certainly true that Truman fired MacArthur, and that he did not use nuclear weapons even though thousands of American troops were being killed or captured.

Truman never explained his decision, but one possible reason is that actually seeing what nuclear weapons do to human beings (which nobody had yet seen when he made his 1945 decision) may have changed his view of them. They were not just another new weapon. They were the ultimate weapon, and they must not be used. And the other reason is obvious.

By late 1950, the United States had between 50 and 100 nuclear weapons — but the Soviet Union had tested its first atomic bomb in the previous year, and by then it already had at least half a dozen of the things. The era of mutual deterrence had arrived.

Truman didn’t know for certain that the Soviet Union would go to war if the U.S. dropped nuclear weapons on China. He would have been fairly certain that the Russians didn’t yet have the ability to drop even one on the U.S., although they could definitely hit America’s allies in Western Europe. But it didn’t matter: Once both sides have nuclear weapons, they get a great deal more cautious.

In the following decades, many military theorists have worked hard to come up with strategies that would make nuclear weapons useful in war, and many scientists and engineers have worked on new techniques and technologies that would achieve the same objective. But nobody has ever had enough confidence in their promises to use even one of these weapons in a war.

The number of nuclear weapons in the world (many of them much more powerful than the Hiroshima and Nagasaki bombs) peaked at around 50,000 in the mid-1980s, and has since fallen to about 15,000. The U.S. and Russia still own 93 percent of them, but seven other countries now have nukes too — and still nobody has used one in war.

It is also true that no great power has fought any other great power directly for 70 years, which is certainly a first in world history. Is this because the two world wars had been so destructive that they created institutions like the U.N. Security Council to avoid another, or because they knew that great-power wars would probably be nuclear wars?

Probably both, but at any rate we’re making progress.

Gwynne Dyer is an independent Canadian journalist, syndicated columnist and military historian based in London.