At 5.29 am on July 16th, 1945, in a test codenamed 'Trinity,' an atomic bomb was detonated at Alamogordo in New Mexico, ushering in the atomic age. The test was of a plutonium bomb, one of two types of nuclear device the Manhattan Project had pursued simultaneously. They knew that a U-235 bomb would work, but it took a long time to enrich enough uranium, so they worked on a more complex plutonium bomb as well. It was a higher tech design in which two different high explosives would surround a plutonium core, producing different speed shock waves that would compress it for long enough to produce sufficient fission in a smaller critical mass. They needed to test it, and on July 16th, the 'gadget' was successfully detonated. Weeks later the US ended World War II with a uranium "Little Boy" atomic bomb on Hiroshima, and then a plutonium "Fat Man" on Nagasaki.
There were about 80,000 dead at Hiroshima, and 40,000 at Nagasaki, and counting those who died subsequently from radiation poisoning, the total might have approached 200,000. On the other hand, a study by William Shockley for the War Secretary estimated that invading Japan would cost 1.7–4.0 million American casualties, including 400,000–800,000 fatalities, and five to ten million Japanese fatalities. Their fanatical defence of Iwo Jima and Okinawa had persuaded Truman that his primary duty was to save American lives, so he authorized the use of the atomic bombs to make an invasion unnecessary.
When the Soviets acquired the bomb in 1949, using information their spies had delivered from the American project, the prospect of war became costly to both sides, resulting in the "nuclear peace" that deterred a European or World War until the Soviet Union collapsed from internal and economic contradictions, and the desire of their captive peoples to be free. The Campaign for Nuclear Disarmament, if successful, would have made war more, not less, likely by reducing the cost to themselves of a Soviet attack. Even today, those who oppose the UK's nuclear deterrent would, if they succeeded, reduce the UK's ability to prevent lunatics in North Korea or Tehran from attacking British interests. The Labour leader, Jeremy Corbyn, is on record with his opposition to both NATO and nuclear arms by Britain and the West.
The Atomic Age never delivered the plentiful and cheap energy it promised. The United States Atomic Energy Commission predicted in 1973 that, by the turn of the 21st century, one thousand reactors would be producing electricity for homes and businesses across the US. It never happened, largely because of public concerns over safety and the costs of decommissioning. Lessons have been learned, though, and it is unlikely that more reactors will be built in areas susceptible to earthquakes and tsunamis, or that poorly trained technicans will be allowed to conduct high risk tests on badly-designed nuclear facilities.
France embraced nuclear technology, and produces 75 percent of its power from nuclear stations. It is also true that Green opposition to nuclear is diminishing. Their scare stories are being replaced by an acceptance that nuclear has a role to play in non-fossil-fuel power production, as part of a non-carbon programme that includes renewables.
The tantalising prospect of non-polluting nuclear fusion still remains only a prospect, despite decades of research. It might happen, just as there might be some new and totally unexpected source of energy developed by research facilities. Meanwhile, over two generations of humankind have learned to live, if somewhat uneasily, with nuclear weapons and nuclear power. It is unlikely that the genie which came out of the bottle in July 1945, can ever be put back, but the world has learned how to achieve some degree of nuclear disarmament, and is still learning how to make nuclear power safer.