The Lessons We Haven't Learned: Seventy-five years after the U.S. atomic bombings of Japan, we remain perched on the precipice of unparalleled catastrophe.

• Author: Caldicott, Helen

My birthday is August 7, sandwiched between the anniversary dates for the bombings of Hiroshima and Nagasaki (on August 6 and August 9, 1945, respectively). I was six years old when the first bomb fell. My course in life was predetermined.

On September 2, 1945, when the local fire siren suddenly blared, my teacher asked, "What is that?" and I knew: The war was over.

It had been a really scary time in Melbourne, Australia, as the Japanese had threatened to invade us. Dad dug an air-raid shelter in our back garden, and the windows were blacked out while the city's searchlights scanned the skies at night.

Elated, I walked home on that lovely sunny afternoon picking flowers along the way. It would be years later before I learned the awful truth about how the war ended.

What rained down on those two Japanese cities seventy-five years ago was destruction on a scale never seen before or since. People exposed within half a mile of the atomic fireball were seared to piles of smoking char in a fraction of a second as their internal organs boiled away. The small black bundles stuck to the streets and bridges and sidewalks of Hiroshima numbered in the thousands.

A little boy was reaching up to catch a red dragonfly with his hand against the blue sky when there was a blinding flash and he disappeared. He turned into gas and left his shadow behind on the pavement, a haunting relic later moved to the Hiroshima Museum. A woman was running while holding her baby; she and the baby were turned into a charcoal statue.

In all, about 120,000 people were killed immediately by the two bombs, and tens of thousands more died later due to radiation exposure.

In 1957, when I was eighteen, I read a book by Nevil Shute, an English novelist who ended up in Australia. On the Beach described how the city of Melbourne awaited a deadly cloud of radiation from a nuclear war that was triggered by an accident in the northern hemisphere, killing everything. Men drank their last gin and tonics in the Melbourne Club while the government dispensed cyanide capsules so parents could kill their children quickly to avoid the agonizing symptoms of radiation poisoning.

At the time, I was in medical school, where I learned about radiation biology--the classic experiments of Hermann J. Muller, who in the 1920s irradiated Drosophila fruit flies inducing genetic mutations and morphological abnormalities. Concurrently, the United States and the Soviet Union were testing nuclear weapons in the...