The United States: Enewetak Atoll, April 30, 1948

In April 1948, the United States had a nuclear monopoly. But the Soviet Union was developing its own nuclear weapons, and the Americans were certainly moving beyond the cumbersome devices they’d dropped on Hiroshima and Nagasaki.

The fifth U.S. nuclear test occurred in 1948, the second of a series of three nuclear explosions at Enewetak Atoll as part of Operation Sandstone. The test, known as “Yoke,” was a 49-kiloton explosion—the largest explosion to date, and a fairly fascinating bomb. Yoke’s implosion design was the first to use highly enriched uranium, something that is of interest to North Korea today. And it was one of the series of tests that confirmed the concept of the “levitated pit,” a design essential to reducing the size of U.S. nuclear weapons so they could fit atop ballistic missiles.

This was a major coup for the U.S. nuclear-weapons program. The bomb the United States had dropped on Nagasaki weighed more than 10,000 pounds. The Sandstone series test opened up a route to reducing that size. A little airspace between the tamper and the fissile core allows the shockwave from the conventional explosives in the bomb to hit harder and more efficiently squeeze the core of fissile material. This design concept was classified for decades, although the physicist Ted Taylor famously spilled the beans when he explained it to the New Yorker journalist John McPhee this way: “When you hammer a nail, what do you do? Do you put the hammer on the nail and push?”

The North Koreans have almost certainly adopted a levitated pit in their own designs.

The Soviet Union: Semipalatinsk (now Kazakhstan), August 23, 1953

The Soviets were behind in 1948, but they were hell-bent on catching up. And going second had its advantages, since Moscow could sit back and let the United States pursue dead ends, while copying approaches that the Americans had bled millions of dollars to confirm.

The fifth Soviet nuclear test—known in the United States as Joe 5, after you-know-who—was of a compact device that could be delivered by aircraft or fitted to a ballistic missile. The Soviet name for the test was Tatyana. This was the Soviets’ first “tactical” nuclear weapon, and it remained in service until 1966. The Soviets were also well on their way toward thermonuclear weapons, having demonstrated the burning of thermonuclear fuel in a so-called Sloika device tested just before Tatyana.

The Soviets were, in many ways, further along by their fifth test than the Americans had been by theirs, which makes sense. They were able to watch the United States to see what might work and what might not. As a result, they had a compact nuclear-weapons design, as well as an early thermonuclear design.

Britain: Montebello Islands, Australia June 19, 1956