On Aug. 6, 1945, the United States dropped an atomic bomb on Hiroshima that obliterated much of the city and sent a mushroom cloud tens of thousands of feet into the air. After 75 years, it continues to cast a shadow over world affairs.

The bombing of Nagasaki three days later was the second and final time the atomic bomb saw use in war. On Aug. 15, Japan announced its surrender.

The weapons caused unparalleled destruction, snuffing out more than 150,000 lives. But those were not the last people to die as a result of nuclear detonations.

In the decades since 1945, the United States, the Soviet Union and at least six other countries set off a total of more than 2,000 nuclear test explosions, which caused tens of thousands of deaths around the world, according to some estimates, along with displacement and environmental degradation that long remained secret and continue to affect communities today.

During World War II, the Soviets began spying on U.S. nuclear efforts and, after the war, a nuclear arms race took shape.

The competition to develop stronger nuclear devices took a human toll. Both governments subjected people at home and abroad to high radiation levels, sometimes with indifference. “Scientists in the 1950s were certainly aware of risks” posed by tests, said Jacob Hamblin, an Oregon State University researcher. “Military demands — not necessarily in wartime — provided a justification for exposing large numbers of people, often under a veil of secrecy.”

No country has conducted more nuclear tests than the United States, which set off its first atomic bomb, in a test code-named Trinity, in New Mexico several weeks before Hiroshima. The barrage of tests that followed wrought a trail of destruction that stretched across continents and decades.

No one has calculated an accurate global body count linked to nuclear testing, or a figure for major U.S. test sites. The United States conducted tests in Nevada, which saw nearly 1,000 nuclear tests, and the Marshall Islands (located between Hawaii and the Philippines), which saw 67. The effects of the testing have often manifested as an increase in cancer rates. Estimates of the number of people who have died as a result of atmospheric tests conducted by the United States from the 1940s through the 1960s range from more than 10,000 to an order of magnitude beyond that.

When tests began in the Marshall Islands in July 1946, U.S. officials relocated inhabitants, promising them they could soon return.

For many of them, that was never possible.