At ten in the morning on Tuesday, Pyongyang time, a mountain in northeastern North Korea shuddered. Seismographs in nearby countries picked up the telltale signs of moving earth. These signals, and their source, suggested to many observers that the tremor was a non-natural event. Not long afterward, the North Korean government announced that it had not only tested a nuclear weapon, as was already suspected, but that it was the country’s first “H-bomb test,” and that it had been “successfully conducted.” The skepticism from Western experts came swiftly. The power of the explosion seemed on par with the largest of North Korea’s previous tests—the equivalent of around ten kilotons of TNT. But hydrogen bombs are typically measured in the hundreds or thousands of kilotons. Was this a bluff, an exaggeration, or something else?

This isn’t the first time that experts have sparred over what is, and what isn’t, an H-bomb. In a long, dull official speech about the budget of the Soviet Union, given in early August of 1953, Premier Georgy Malenkov announced to the world that the “the U.S. has no monopoly in the production of the hydrogen bomb.” His claim was greeted with a flurry of coverage in American newspapers, and with some incredulity among American politicians and nuclear experts. Edwin Johnson, a senator from Colorado, called it “manufactured propaganda.” He added: “I would take it with a grain of salt.” But then, three days later, the Soviets set off something big. Pravda, the official Communist Party newspaper, crowed that a “mighty thermonuclear reaction” had taken place. Perhaps the Soviets had the H-bomb after all.

U.S. Air Force planes had been sniffing around the borders of the U.S.S.R. since late 1948, looking for the residues of nuclear detonations. The dust that is left behind after an explosion can reveal a good deal about how a bomb works, with different radioactive isotopes signalling different processes. For instance, it is possible to discern whether a nuclear reaction consisted of fission (the splitting of heavy atoms) or also of fusion (the merging of light atoms), whether the fissile material was uranium or plutonium, and even, if both sorts of reaction took place, whether they began physically near or apart from each other. The radioactive remains of the Soviet test, which the C.I.A. had dubbed Joe 4, were duly picked up and sent to various secret laboratories. The raw data was then passed on to a panel of A-list nuclear physicists, headed by the future Nobel Prize winner Hans Bethe, which was tasked with interpreting it. In their classified report, which was finished that September, Bethe’s scientists took issue with the characterization of the Soviet weapon as a hydrogen bomb. The device had, in truth, produced “a substantial thermonuclear reaction,” and its explosive power—equal to four hundred thousand tons of TNT—was “certainly enough to cause concern.” But it wasn’t an H-bomb, at least as the panel construed the term.

What was it, then? To answer that question requires going back a little further, to the American weapons program of the nineteen-forties. The initial idea for the H-bomb was vague. Before the attacks on Hiroshima and Nagasaki had taken place, before the United States had even built a working atomic weapon, Enrico Fermi suggested to Edward Teller, his colleague at the government’s laboratory in Los Alamos, New Mexico, that it might be possible to use a fission reaction to jump-start fusion. That idea—elements from one end of the periodic table (plutonium and uranium) manipulating an element at the other end (hydrogen)—remained a constant feature of the weapon. But Teller and his colleagues had trouble making it work in practice. Between the end of the Second World War and 1951, they developed four candidates for what might be called a hydrogen bomb. Only one, in the end, became the definitive design.

The first concept was known as the Super, eventually differentiated as the Non-Equilibrium or Classical Super. The idea was to take a very large fission bomb and attach a tube of hydrogen isotopes (deuterium and tritium) to it. The heat of the initial explosion would start a reaction at one end of the tube that would continue down its length. If the design could be made to work, the power of the weapon would depend only on how long the tube was—theoretically limitless, although dropping such a thing from an airplane might prove cumbersome. The problem, though, was that the fusion reaction lost energy too quickly and stopped. As a result, the Classical Super was shelved, in 1950. ...