In an echo of that understanding, the number of Americans who say the US was justified in using the atomic bomb has dropped to 56 percent, according to a Pew Research Center poll from this spring. And while that still represents a majority – and although a sense of justification is not the same as outright approval – the figures show a clear decline, Mr. Stokes says.

“I think over the last 70 years, people have become more aware of what the nuclear age is and the consequences of it,” Allan Winkler, a distinguished professor of history at the Miami University of Ohio, says in a phone interview.

That awareness is vital, Prof. Winkler says, particularly in light of discourse around the nuclear deal that six world powers, including the United States, struck with Iran last month. As contested as its contents are, he adds, the deal represents continued efforts at preventing both the spread of nuclear weapons and the resurgence of the sense of fear and urgency that led then-President Harry Truman to unleash the bomb in the first place.