August 10th, 1945. The newspapers say the war is almost over. Hiroshima is gone. Nagasaki is burning. Commentators talk about the dawn of the atomic age. Like the story is already finished. It is not. In Washington, DC, General George Marshall, the army’s chief of staff, has just received an urgent memo from the man behind the Manhattan project, Lieutenant General Leslie Groves.
It explains that another plutonium core is being cast at Los Alamos. If Japan refuses to surrender, a third atomic bomb will be ready within days. Shipping can be arranged and a target can be chosen. As if dropping nukes has become a regular thing. But instead of hitting Japan, the third bomb would go on to kill Americans.
This was the moment the atomic bomb stopped being a last resort and started becoming the way modern power really works. Before Hiroshima, the atomic bomb was still an experiment. In July 1945, at a site in the New Mexico desert called Trinity, the first plutonium implosion device was fired.
For a moment, the desert lit up like daylight, and the scientists watching understood that their equations could now erase a city in a single blow. Within weeks, that test became a production schedule. Loss Alamos was no longer trying to prove that a bomb could work. It was turning that design into a series of weapons. First came the uranium bomb that would be called Little Boy.
Then the plutonium weapon that destroyed Nagasaki. Fat Man. Behind those two, more plutonium cores moved through the system. Each one a man-made disaster waiting to be used. By early August 1945, two bombs had already been used, but the machine that made them had not slowed down. Reactors at Hanford were still producing plutonium, and at Los Alamos, it was still being shaped into dense metal spheres.
Back in 1942, the question had been simple. Could the United States build an atomic weapon before its enemies did? By the late summer of 1945, that question had quietly changed. Inside the American High Command, people were now thinking about how many of these weapons they might need and how quickly they could be ready.
To see what that means, look at what is happening in Japan at the same time. On August 6th, Hiroshima is destroyed by a single bomb. On August 8th, the Soviet Union declares war on Japan. On August 9th, Nagasaki is hit. That same day in Manuria, the Red Army launches Operation August Storm, a huge offensive against the Japanese Quantum Army.
Within days, Japanese lines begin to fall apart. In Tokyo, this forces a crisis at the very top. Emperor Hirohito can see that the combination of atomic attacks and Soviet entry into the war has made any hope of victory impossible. He wants to end the fighting. His senior military leaders do not agree. Some are ready to fight on the beaches and in the streets, convinced that if they can cause enough casualties in an American landing, the United States will ease its terms.
Others look at the burnedout cities, the lack of fuel, and the Soviet advance through Manuria, and conclude that the war is already lost. In Washington, the picture looks different. American planners are working on Operation Downfall, an invasion of the Japanese home islands in two stages. Operation Olympic on Kyushu and Operation Coronet on Honshu near Tokyo.
Estimates for how many people might die in such an invasion vary widely. Some forecasts speak of several hundred,000 American dead and wounded. Others, depending on how long the fighting lasts, push total Allied losses toward a million. For President Harry Truman, this turns into a brutal calculation. Every extra day that Tokyo delays surrender means more American servicemen killed in the Pacific and more Japanese civilians dying under conventional bombing.
It is in that setting that the orders go out to use atomic weapons against Hiroshima and Nagasaki. They are not seen simply as statements. They are meant to hit Japan’s leaders so hard that surrender looks better than invasion. Yet even after two cities have been destroyed, the debate inside Tokyo is not settled. From the American side, a hard question starts to creep in.
What if these two strikes still do not end the war? At that point, more weapons already on the way become crucial. The people drawing up plans in Washington know they do not have to reinvent the bomb if Japan continues to hold out. They only have to wait for the next completed core to come through the system.
That shift from wondering if such a weapon could ever be built to asking when the next one will be ready is what gives that next core, the one prepared for a third attack, its deeper historical meaning. On August 10th and 11th, the war is balanced on a knife edge. In Washington, cables and reports are flying. American intelligence is trying to read the mood in Tokyo.
Diplomats are listening for any clear sign of the emperor’s position. Military planners are watching the Soviet advance in Manuria and asking how far the Red Army intends to go. At the same time, the machinery that built the first two bombs is still running. At Los Alamos, another plutonium core is being finished.
The parts for a third weapon are ready to be brought together, flown to the Pacific, and handed to a B29 crew if the order comes. For the men at the top, this is no longer a theoretical discussion about what is possible. It is a question of what to do with a weapon that is almost ready. General Leslie Groves makes it clear to Washington that another device can be prepared on schedule.
General George Marshall has already passed that information to President Harry Truman and added a condition in his own hand that no further bomb is to be used on Japan without explicit presidential approval. For a man who only learned about the project after Roosevelt’s death, Truman now sits in a unique place. He is the single decision point between a core on a workbench and a fireball over a city.
That is new. The bomb is no longer just a year terrifying scientific breakthrough. It is something that must move through a chain of command. Something that can be held back or used by order of one office. In the months and years that follow, that basic idea will grow into a whole system of control around nuclear weapons.
But in August 1945, it is still taking shape. Japan surrenders before the third weapon is ever assembled for combat. The plutonium core meant for it never leaves American soil. No bomber carries it across the Pacific. No crew trains to drop it on a Japanese city. Instead, the core stays at Los Alamos and is given a different job. It becomes a test piece.
Physicists use it to study how close they can push a mass of plutonium toward a chain reaction without triggering a full scale explosion. On paper, the procedure sounds straightforward. You place neutron reflecting material around the core, bring it closer bit by bit, and measure how the reactions build as you approach the danger point.
In reality, it means working within inches of a substance that can suddenly pour out a lethal burst of radiation if you make a mistake. In August 1945, a young physicist named Harry Daglian is running one of these tests. Late at night, in a concrete building, he stacks heavy tungsten carbide bricks around the core. One brick slips from his hand and falls into place.
In that instant, the setup crosses the line into super critical. The core starts to run away. The room is flooded with invisible radiation. Daglian reacts on instinct. He knocks the bricks away with his bare hands, breaking the configuration and stopping the reaction. He saves the others nearby. He cannot save himself. Over the next weeks, his body fails under massive radiation damage, and he dies in September, the first person killed by this core.
The war is over, but the third bomb has claimed its first victim. The work at Los Alamos does not stop. The experiments resume under new rules, at least on paper. In May 1946, another physicist, Luis Slutan, demonstrates a different kind of test for colleagues. This time, the reflector is a burillium shell. The core sits on a stand.
A half shell is lowered over it with only a small gap left open. Slotin holds that gap with the tip of a screwdriver, raising and lowering the shell to show how the readings change. It looks routine, almost casual. Then the screwdriver slips. The two halves of the shell close. The gap disappears. For a brief moment, the core goes prompt critical.
Witnesses see a blue flash as the air around the assembly is hit by intense radiation. Slotin yanks the shell away with his hands, breaking the setup. Everyone in the room receives a heavy dose. Slotin, closest to the core, receives the most. Over the next 9 days, he suffers the full course of acute radiation sickness and then dies.
The core that was once intended for a third strike on Japan has now killed two of the men who worked with it. Only after these accidents does it gain the name that will stick in them. History of the bomb, the demon core. The symbolism is hard to miss. This device was built as part of a plan to force another enemy city to its knees.
Instead, it kills its own makers inside a laboratory. It shows in the starkkest possible way that there is no such thing as a completely safe nuclear stockpile. Every core you cast carries risk from the moment it exists, whether it ever leaves your borders or not. The response in Washington is not to shut the program down.
It is to control it more tightly, to keep building weapons under stricter rules, and to start thinking about how many might be needed in a future conflict. While the demon corps sits on tables in New Mexico, planners are already beginning to ask a new set of questions. If there is another great war, how many of these weapons will we need on the first day? And who are they really meant to deter? In the late 1940s, the United States holds something no other country has, working atomic weapons.
From 1945 until the Soviet test in 1949, America is the only nuclear power on Earth. No one in Washington thinks that monopoly will last. leaders know the Soviets are racing to build their own bomb. They know the Red Army is still huge and sitting in the middle of Europe and they know that if there is another great war, it will not look like 1939 to 1945.
It will take place under the shadow of nuclear weapons. In that world, the logic born with the third core grows into something much larger. During the war, the bomb had been seen as a brutal shortcut, a way to break Japan without an invasion. After the war, the question slowly shifts. If the next enemy is the Soviet Union, how many bombs would you need on the first day of a new conflict? Early plans imagine using a few dozen weapons against key Soviet factories, airfields, and bases.
As the late 1940s turned into the 1950s, those numbers rise. One later plan, known as Operation Dropshot, sketches a possible war with the Soviet Union around 1957. In that scenario, the United States and its allies would use hundreds of nuclear weapons on the order of 300 bombs against roughly 200 targets across the Soviet Union and Eastern Europe alongside tens of thousands of conventional bombs on railways, oil sites, ports, and troop concentrations.
This is not a vision of the bomb as a last resort. It is a plan to open World War II with a nuclear barrage. In just over a decade, Western military thinking moves from wondering if a third bomb might be needed to force Japan to surrender to planning for hundreds of bombs aimed at the Soviet Union in the first days of a new war.
This is nuclear brinkmanship in its pure form. You do not want to fight that war. You want the threat of it and the scale of damage you can promise to shape your opponent’s choices. The mental framework that allows that kind of planning rests on the habits formed in 1945. Once you start casting calls on a schedule, and once senior officers are used to reading memos that say the next one will be ready by this date, the bomb is no longer a one-time horror.
It has become a series. After that, moving from 3 to 30 to 300 is not a leap in imagination. It is a matter of time, industry, and political will. The third core itself does not survive as a single object. After the slot accident, it is taken out of direct hands-on experiments. Eventually, it is melted down and its plutonium is reused in other tests.
No city ever sees a mushroom cloud from that specific core, but its influence runs through the early nuclear age. It leaves behind three lessons that rarely appear in simplified histories. The first is about control. The insistence typed and then underlined by hand that no further bomb be used on Japan without presidential approval grows into a wider system of nuclear command and control.
A set of rules that says these weapons sit in a special category and require a unique chain of decisions. Out of that idea come the tools and rituals that later surround the Cold War presidency. From the nuclear briefcase to coded launch orders and emergency procedures. The second lesson is about quantity. Once you plan for a third bomb, you have already stopped treating atomic weapons as one-off miracles.
They become items you can stockpile and assign in war plans alongside ships and aircraft. That way of thinking opens the door to operations that assume hundreds of warheads and to the vast arsenals that the United States and the Soviet Union eventually build with tens of thousands of nuclear weapons between them. The third lesson is about risk.
The demon core never turned a foreign city into ash, but it still killed. It killed the men who worked with it. It contaminated the rooms where it was tested. It showed that the danger begins the moment you create the material. And that the risk spreads into laboratories, transport routes, storage sites, submarines, and missile fields, whether a weapon is ever used in anger or not.
When people talk about the start of the atomic age, they often like simple numbers. Two bombs, two cities, the war ends. The world is shocked. The real story is more tangled. In August 1945, while Hiroshima and Nagasaki were still burning, a third bomb was already in preparation. In Washington, that core helped push generals and presidents toward a new kind of decision-making.
In New Mexico, it killed the people who pushed it too far. In the planning rooms of the early Cold War, the logic it embodied was multiplied into hundreds of weapons aimed at another superpower. The true legacy of that third bomb lies less in what it destroyed than in what it taught. It taught that nuclear weapons could be scheduled, that they could be counted, stored, and moved around as bargaining chips.
That the threat of their use could reshape global politics even if they were never dropped again. We remember the huge clouds in the sky because we can see them. What we do not see as easily is the timet behind them. The next core, the next weapon, the next set of orders waiting in a file. The third bomb never created its own mushroom cloud, but in quiet ways, it may be the most important of all, because once you build it and put it on a calendar, the question stops being whether humanity can make such weapons.
