Initially, purple spots appeared on their bodies, their hair fell out, and they developed high fevers, infections, and swollen and bleeding gums. Later, cancer rates surged. The survivors, known as hibakusha, lived in constant fear of illness and death.

The United States suppressed this part of the story. In the fall of 1945, high-level American officials rebutted news reports of deaths from radiation exposure. For years to come, the occupation authorities censored news accounts, photographs, scientific research and personal testimonies about the attacks.

To counter growing criticism of the bombings, American leaders established a narrative that the bombings had ended the war and saved up to 1 million American lives by preventing an invasion of Japan. (These postwar casualty estimates were far higher than pre-bomb calculations.) Most Americans accepted this narrative.

Few Americans know much about Nagasaki. A center for trade in the late 1500s, it was the vanguard for the nation’s early modernization and the hub for Catholic missionary outreach. When Japan officially banned Christianity in 1614, then closed its borders to outside contact from the 1630s to the late 1850s, Nagasaki alone was allowed to continue limited international trade, providing the city’s growing population exposure to Asian and European arts, science and literature. Nagasaki continued to thrive after Japan re-established diplomatic relations with the West, becoming the third largest shipbuilding city in the world. Christians who had long hidden their faith re-emerged, and Nagasaki became the home of the largest Catholic church in East Asia. An estimated 10,000 Catholics died in the 1945 bombing.

Sumiteru Taniguchi, 16, was delivering mail on his bicycle in the northwestern corner of the city when the force of the explosion hurled him into the air. Even from a mile away, the searing heat instantly disintegrated his cotton shirt and burned the skin off his entire back and one arm. Three months later, he was finally taken to a naval hospital 22 miles north of the city, where he lay on his stomach for more than three years, begging the nurses to let him die. Later, after he had learned to sit, stand and ultimately walk again, Mr. Taniguchi seethed in anger at what he believed was the unnecessary nuclear devastation of his city and its people.

Over the past 70 years, Mr. Taniguchi and tens of thousands of other hibakusha have navigated punishing injuries, late-onset radiation-related illnesses, and haunting fears that they would pass on genetic disorders to their children and grandchildren. Many never speak about their atomic bomb experiences, even within their families. In a remarkable act of resilience, however, Mr. Taniguchi and a small number of hibakusha made the very personal choice, some as early as the mid-1950s, to speak publicly about their survival.

They do not tell their stories to promote Japan’s victimization, or to minimize the attack on Pearl Harbor, or the suffering and deaths of Asian civilians and Allied military personnel at the hands of brutal Japanese soldiers. Rather, they speak to eliminate ignorance about the realities of nuclear war and to eradicate nuclear stockpiles across the globe.

The official narrative remains the dominant opinion of most Americans. In that story, Nagasaki fades in memory; we should not let it. Our time to understand the survivors’ experience of nuclear war is running out. Only they can tell us what it was like, and their lives are coming to an end.