James Conant, the American scientist who played a key role in the creation of the first atomic bomb, never regretted dropping it on Hiroshima, Japan, at the end of World War II. But he was afraid of a nuclear standoff, similar to the one brewing today between the United States and North Korea.

Now Conant's granddaughter, the best-selling author Jennet Conant, is writing about his life and career in the new book, "Man of the Hour: James B. Conant, Warrior Scientist," published by Simon & Schuster, a division of CBS.

"This was his nightmare scenario, that we would have this enormous arms race and that it would just increase sort of unabated, and that we would inevitably find ourselves, as he said, 'like two gunmen with itchy trigger fingers,'" she told CBS News correspondent Tony Dokoupil.

James Conant wasn't the kind of person given to fear or exaggeration, and yet, "My grandfather was really so terrified of a nuclear conflict… I think the idea that mutually assured destruction would have held for almost 70 years would have surprised him," Jennet said.

In the late '30s, Conant was a brilliant chemist, a veteran of poison gas production during World War I, and a successful president of Harvard University. But his life changed course after Albert Einstein warned the White House about the potential for "extremely powerful bombs."

James B. Conant

That triggered a desperate race to build a nuclear weapon before Hitler's Germany could do so. The task of winning fell to Conant and a secret team of scientists.

"He was the supervisor of everything that happened in terms of the bomb's development," Jennet said.

What happened in the summer of 1945 was the first open-air test of a nuclear weapon – a blast so shocking that Conant, from a nearby bunker, was sure the team had miscalculated.

"He thought, in that moment, the world is over," Dokoupil said.

"He did. Terrifying. Absolutely terrifying," Jennet Conant said.

Three weeks later, Hiroshima and then Nagasaki became the targets of the only wartime uses of a nuclear weapon.

"We have spent more than $2 billion on the greatest scientific gamble in history – and we have won," then-President Harry S. Truman said.

Hundreds of thousands died in the blasts and their aftermath.

"People always ask me, you know, 'Did they feel guilty?' They really felt that they had done the right thing in building that bomb. It did shorten the war. It did save lives," Jennet said.

Conant and his colleagues warned of the need to control the bomb by sharing the science and striking a global deal to curtail production. But the scientists were overruled, and today, nine countries have confirmed or suspected nuclear arsenals, including the rogue regime of North Korea, which is testing missiles and threatening nuclear destruction.