Initial nuclear bomb designs used a gun-type method, slamming together two pieces of subcritical U-235 to create an explosion, though these were relatively unsafe and prone to accidental detonation. Implosion-type bombs came next, featuring a fissile core surrounded by an explosive lens that, when ignited, squeezed the core material into a supercritical mass. Implosion-type bombs gave scientists more control over detonation and allowed them to use plutonium as the fissile material.

The US detonated the first nuclear weapon, an implosion-type plutonium bomb, at 5:29 AM on July 16th, 1945. It exploded with the energy of 20,000 tons of TNT, and the blast immediately transformed the desert sand into radioactive green glass.

Enrico Fermi witnessed the detonation.

"I had the impression that suddenly the countryside became brighter than in full daylight," he wrote. "I subsequently looked in the direction of the explosion through the dark glass and could see something that looked like a conglomeration of flames that promptly started rising. After a few seconds, the rising flames lost their brightness and appeared as a huge pillar of smoke with an expanded head like a gigantic mushroom that rose rapidly beyond the clouds, probably to a height of 30,000 feet. After reaching its full height, the smoke stayed stationary for a while before the wind started dissipating it."

Six days later, President Harry S. Truman wrote the following entry in his diary:

"We have discovered the most terrible bomb in the history of the world. It may be the fire destruction prophesied in the Euphrates Valley Era, after Noah and his fabulous Ark. ... This weapon is to be used against Japan between now and August 10th. I have told the Sec. of War, Mr. Stimson, to use it so that military objectives and soldiers and sailors are the target and not women and children."

On August 6, 1945, the US dropped a uranium gun-type bomb called Little Boy on Hiroshima, Japan. Truman gave the order.

The next day, he received the casualty reports: 80,000 people were killed and 70,000 injured, most of them civilian men, women and children. The US had deleted Hiroshima from the face of the Earth.

Journalist John Hersey traveled to the region one year later to gather first-hand accounts of the blast and aftermath. His resulting book, Hiroshima, describes the scene as follows:

"The night was hot, and it seemed even hotter because of the fires against the sky, but the younger of the two girls Mr. Tanimoto and the priests had rescued complained to Father Kleinsorge that she was cold. He covered her with his jacket. She and her older sister had been in the salt water of the river for a couple of hours before being rescued. The younger one had huge, raw flash burns on her body; the salt water must have been excruciatingly painful to her. She began to shiver heavily, and again said it was cold. Father Kleinsorge borrowed a blanket from someone nearby and wrapped her up, but she shook more and more, and said again, 'I am so cold,' and then she suddenly stopped shivering and was dead. Mr. Tanimoto found about twenty men and women on the sandspit. He drove the boat on to the bank and urged them to get aboard. They did not move and he realized that they were too weak to lift themselves. He reached down and took a woman by the hands, but her skin slipped off in huge, glove-like pieces."

Three days later, the US dropped a second nuclear bomb on Nagasaki, Japan. This one was designed to be even more powerful than the first -- it was an implosion-type plutonium bomb. The region's rolling hills helped contain the explosion, but it still killed at least 40,000 people and injured 60,000 more. Just 150 of those killed were military personnel.

It's unclear whether Truman actually ordered the second bombing.

The day after the US attacked Nagasaki, the president received a memo from Maj. Gen. Leslie Groves outlining a third nuclear attack in Japan. It was scheduled for August 17 or 18, just a week away.

This bombing never happened.

Truman's Army chief of staff, George Marshall, hand-wrote a response to Groves directly on that memo, reading, "It is not to be released over Japan without express authority from the President."

This is how the US nuclear hierarchy was established. Truman and the military wrestled over control of the nation's nuclear stockpile, as did presidents after him. However, after Truman's message to Groves, the ultimate power to launch a nuclear weapon never left the Oval Office.

Proliferation

For years after World War II, the US was secure in its position as the only nuclear power in the world. And then, in 1949, the Soviet Union detonated its own nuclear bomb, a copy of the implosion-type plutonium warhead dropped on Nagasaki. The new atomic arms race was on. Today, there are more than 15,000 nuclear warheads hidden around the world. The United States has 6,800, while Russia, our foil in the Cold War, has an estimated 7,000. The remaining few hundred warheads are distributed among France, China, the UK, Pakistan, India, Israel and North Korea.

Of these nine nuclear powers, the US is the only country without a system of checks and balances at the top of its launch process. The president of the United States is the only person in the world with the sole authority to launch a nuclear strike.

"We're the only country that has this bizarre arrangement where anybody in the White House who's officially our president can do the launch," says Richard Rhodes, historian and author of The Making of the Atomic Bomb. "It's really quite terrifying. I've noticed that there's recently been sudden movement in Congress to move that power back where the Constitution puts it, which is in Congress. It was moved to the president because they were concerned about not having any time to deliberate."

Time is a major factor in the nuclear-launch process. In the event of a pre-emptive strike on the US, the president may have only seconds to authorize a response, and the military can have nuclear warheads in the sky within minutes.

In 1973, Harold Hering was training to be a minuteman, one of the people who would turn the key and actually launch a nuclear warhead if the order came down. He was nearly done with a months-long training program when he asked the following question of his superior officers: "How can I know that an order I receive to launch my missiles came from a sane president?"

Hering was discharged from the Air Force that same year. He never got an answer.

As it turns out, the US's nuclear-launch systems take Hering's hesitation into account.

In five minutes How the US would go about launching a nuclear strike, as former minuteman Bruce Blair explains to Bloomberg: The president considers a strike.

He meets with top military and civilian advisers, either in the Situation Room at the White House or on a secure phone line. This meeting is as long -- or as brief -- as the president wants; it may last for just 30 seconds.

The president decides to strike. At this point, advisers can resign in protest or otherwise attempt to sway him against a nuclear attack, but the Pentagon will carry out the command without question.

The Pentagon confirms the order by reading a "challenge code," two letters with a matching pair only the president should know. The president reads the matching pair off of the "biscuit," a laminated card he or an aide carries at all times.

The Pentagon sends the order to every worldwide command center and launch crew. It's an encrypted message roughly the length of a tweet.

Launch crews open locked safes containing sealed authentication-system codes and compare them with the order.

A military submarine or land-based launch crew authenticates the order. It takes 15 minutes to launch a nuclear warhead from a submarine, authenticated by the captain, executive officer and two other crew members. The land-based method is slightly more complicated: Five underground launch teams, spread miles apart, authenticate the order and re-target their missiles. They unlock the payloads. At the predetermined time, all five crews turn their keys simultaneously, sending five "votes" to the launch system. It takes just two "votes" to actually launch a nuclear warhead. This entire process can take as few as five minutes.

Actually launching a nuclear strike is not something most people take lightly, no matter their training or patriotism, and the potential for mutiny is built directly into the US's systems. Advisers can resign in protest and three entire launch crews can decide not to turn their keys, but the bombs will still be deployed. "It's hard to imagine getting back to world-scale war unless the two main superpowers do something so incredibly stupid that they've managed not to do in the last 70, 75 years -- which brings us to Donald Trump," Rhodes says.

Trump is a wild card when it comes to military force and nuclear authority. He's ordered non-nuclear bombings based on fickle gut reactions, he discusses national security threats in public, and he's displayed a cavalier attitude toward nuclear weaponry in general. During the 2016 presidential campaign, Trump suggested more countries should have nuclear weapons, and in February, he dismissed a critical disarmament treaty between the US and Russia as a "one-sided deal."

Plus, Trump has shaken up the US chain of command, giving his daughter and son-in-law top security clearance and a range of responsibilities. It's unclear who has Trump's ear at any given time, or whom he would invite into the Situation Room for advice on a nuclear strike.

Rhodes says it's no wonder countries like North Korea are wary.

"These guys are really, really jumpy about being attacked, and I can understand why they are with someone like Trump in the presidency," he says.

Of course, Trump isn't the first wild card the nuclear world has seen. Hell, he's not even the only one alive today: Kim Jong-Un is wildly unpredictable and increasingly desperate, and Vladimir Putin is trying his damnedest to prove Russia's might on a global scale. There have been numerous close calls throughout the years when Russia or the US nearly pushed that big red button, but most of these were avoided thanks to the supreme caution of commanding officers.

"During the end of the Nixon regime, the Secretary of State and the Secretary of Defense had concurred that if Nixon ordered a nuclear attack on somebody, it would not happen," Levenson says. "They were prepared to not do what he ordered. Today, anybody who thinks they can predict what our current government is going to do is kidding themselves."

The Russian military even conducted a social experiment on its own officers during the Cold War, when top brass wanted to see how prepared their intercontinental ballistic missiles were for an actual strike.

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Milton Levenson on Russia's wartime experiment.

Soviet authorities planted stories in the press saying disarmament negotiations with the US were not going well, and they warned launch crews to refuel their rockets in preparation for an attack. A week later, after manufacturing all of this tension in its missile silos, officials sent out an urgent message ordering crews to roll back their shielding doors and get ready to launch.

"Not a single commander of a single Russian base did that," Levenson says. "Every one of them got on the phone to try to find some senior person to find out if this was for real. That really shook up, as you can imagine, the Kremlin because if it had been a real event they would not have been able to respond."

"It was one of the sausage-grinding operations where it never stopped. They built an enormous number of weapons that no one in his right mind, or even wrong mind, would ever think of using."

The US and Russia own 90 percent of the world's nuclear warheads, and the size of their arsenals means they dominate the international conversation. During the Cold War, Rhodes says the USSR built roughly 95,000 nuclear weapons and the US built around 45,000.

"The Soviets had a system that they'd always had since Communism took over, that everybody was supposed to crank out 110 percent every year, every factory, and so forth," he says, noting supply concerns also helped ramp up Soviet production. "It was one of the sausage-grinding operations where it never stopped. They built an enormous number of weapons that no one in his right mind, or even wrong mind, would ever think of using."

These stockpiles have shrunk significantly, but the US and Russia still have thousands of warheads apiece. The weapons themselves have evolved as scientists continue to experiment with fissile materials and the enormous amounts of energy of contained in a nuclear blast.

Boosted fission bombs, for example, can more than double the energy released by a traditional implosion-type weapon. But that's nothing: The most powerful nuclear bomb ever tested, Russia's Tsar Bomba, was a three-stage thermonuclear device that exploded with the energy of 57 megatons of TNT when it was detonated in 1961. Staged thermonuclear weapons can, in theory, stack multiple radiation implosions, each one significantly increasing the power of the blast without changing the size of the warhead much. This is one way to build a bomb capable of literally destroying the world.

During the Cold War, President Ronald Reagan greenlit Project Excalibur, a program spearheaded by notorious nuclear physicist Edward Teller. The plan was to build an in-orbit X-ray laser powered by a nuclear bomb and capable of destroying any Soviet missile that might be fired. The project became known as "Star Wars" and it was, in fact, more fiction than science.

"The only little problem, of course, was that if you explode a nuclear weapon in near-orbit, you'd get what's called an electromagnetic pulse that would burn out the electronics on every computer on that side of the Earth, and every automobile ignition, and every circuit on a power plant," Rhodes says. "You would basically take out the electrical system for the entire hemisphere."