At 5:30 a.m. on July 16, 1945, a light brighter than the sun radiated over New Mexico. The fireball annihilated everything in the vicinity, then produced a mushroom cloud that billowed more than seven miles high.

In the aftermath, the scientists who had produced the blast laughed and shook hands and passed around celebratory drinks. Then they settled into grim thought about the deadly potential of the weapon they had created. They had just produced the world’s first nuclear explosion. (Here's what happened that day in the desert.)

The test, code-named “Trinity,” was a triumph; it proved that scientists could harness the power of plutonium fission. It thrust the world into the atomic age, changing warfare and geopolitical relations forever. Less than a month later, the U.S. dropped two nuclear weapons on Hiroshima and Nagasaki, Japan—further proving it was now possible to obliterate large swaths of land and kill masses of people in seconds.

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In August 1945, the United States decided to drop its newly developed nuclear weapons on the Japanese cities Hiroshima and Nagasaki in an attempt to end World War II. In this photograph, an unidentified man stands next to a tiled fireplace where a house once stood in Hiroshima on Sept. 7, 1945.

Scientists had been trying to figure out how to produce nuclear fission—a reaction that happens when atomic nuclei are split, producing a massive amount of power—since the phenomenon’s discovery in the 1930s. Nazi Germany was first to try to weaponize such energy, and word of its efforts leaked out of the country along with political dissidents and exiled scientists, many of them German Jews.

In 1941, after emigre physicist Albert Einstein warned President Franklin Delano Roosevelt that Germany might be trying to develop a fission bomb, the United States joined the first nuclear arms race. It launched a secret atomic research project, code-named the Manhattan Project, bringing together the nation’s most eminent physicists with exiled scientists from Germany and other Nazi-occupied countries.

The project was carried out at dozens of sites, from Los Alamos, New Mexico, to Oak Ridge, Tennessee. Although it employed an estimated 600,000 people over the life of the project, its purpose was so secret that many of the people who contributed to it had no sense of how their efforts contributed to the larger, coordinated goal. Researchers pursued two paths toward a nuclear weapon: one that relied on uranium and another, more complex path, that relied on plutonium.

After years of research, the Manhattan Project made history in 1945 when the test of “the gadget,” one of three plutonium bombs produced before the end of the war, succeeded. The U.S. had also developed an untested uranium bomb. Despite the obvious potential of these weapons to end or alter the course of the ongoing World War II, many of the scientists who helped develop nuclear technology opposed its use in warfare. Leo Szilard, a physicist who discovered the nuclear chain reaction, petitioned the administration of Harry S. Truman (who had succeeded Roosevelt as president) not to use it in war. But his pleas, which were accompanied by the signatures of scores of Manhattan Project scientists, went unheard.

On August 6, 1945, a B-29 “superbomber” dropped a uranium bomb over Hiroshima in an attempt to force Japan’s unconditional surrender. Three days later, the U.S. dropped a plutonium bomb, identical to the Trinity test bomb, over Nagasaki. The attacks decimated both cities and killed or wounded at least 200,000 civilians. (For those who survived, memories of the bomb are impossible to forget.)

Japan surrendered on August 15. Some historians argue the nuclear blasts had an additional purpose: to intimidate the Soviet Union. Without a doubt, the blasts kicked off the Cold War.

Soviet leader Joseph Stalin had already green-lit a nuclear program in 1943, and a year and a half after the bombings in Japan, the Soviet Union achieved its first nuclear chain reaction. In 1949, the U.S.S.R. tested “First Lightening,” its first nuclear device.

Ironically, the United States leadership believed that building a robust nuclear arsenal would act as a deterrent, helping prevent a third world war by showing that the U.S. could crush the U.S.S.R., should it invade Western Europe. But as the U.S. began investing in thermonuclear weapons with hundreds of times the firepower of the bombs it used to end World War II, the Soviets followed on its heels. In 1961, the Soviet Union tested the “Tsar Bomba,” a powerful weapon yielding the equivalent of 50 megatons of TNT and producing a mushroom cloud as high as Mount Everest.

“No matter how many bombs they had or how big their explosions grew, they needed more and bigger,” writes historian Craig Nelson. ”Enough was never enough.”

A Nuclear Reality

Astronomer Carl Sagan reflects on the proliferation of nuclear weapons.

As additional countries gained nuclear capacity and the Cold War reached a fever pitch in the late 1950s and early 1960s, an anti-nuclear movement grew in response to a variety of nuclear accidents and weapons tests with environmental and human tolls.

Scientists and the public began to push first for a ban on nuclear testing and then for disarmament. Einstein—whose initial warning to Roosevelt had been designed to prevent nuclear war, rather than set it in motion—was among them. In a 1955 manifesto, the physicist and a group of intellectuals pleaded for the world to abandon its nuclear weapons. “Here, then, is the problem which we present to you, stark and dreadful and inescapable,” they wrote. “Shall we put an end to the human race; or shall mankind renounce war?”

The urgent issue went unresolved. Then, in 1962, reports of a Soviet arms build-up in Cuba led to the Cuban Missile Crisis, a tense standoff between the U.S. and U.S.S.R. that many feared would end in nuclear catastrophe.

In response to activists’ concerns, the U.S. and the U.S.S.R. (and later Russia) signed a partial test ban treaty in 1963, followed by a nuclear nonproliferation treaty in 1968, and a variety of additional agreements designed to limit the number of nuclear weapons.

Nevertheless, in early 2020 there were an estimated 13,410 nuclear weapons in the world—down from a peak of around 70,300 in 1986—according to the Federation of American Scientists. The FAS reports that 91 percent of all nuclear warheads are owned by Russia and the U.S. The other nuclear nations are France, China, the United Kingdom, Israel, Pakistan, India, and North Korea. Iran is suspected of attempting to build its own nuclear weapon.

Despite the dangers of nuclear proliferation, only two nuclear weapons—the ones dropped on Hiroshima and Nagasaki—have been deployed in a war. Still, writes the United Nations Office for Disarmament Affairs, "The dangers from such weapons arise from their very existence.”

Seventy-five years after the Trinity test, humanity has thus far survived the nuclear age. But in a world with thousands of nuclear weapons, constantly changing political alliances, and continued geopolitical strife, the concerns raised by the scientists who birthed the technology that makes nuclear war possible remain.