North Korea has made a nuclear weapon small enough to fit on a missile. How worried should you be?

North Korea is making progress, showing it can put together competent teams of scientists and solve technical problems, but it is far from proving that it is capable of launching a punishing nuclear strike on the U.S., according to U.S. weapons experts.

Making a miniature nuclear weapon that has a large explosive force involves a lot of scientific and engineering know-how.

The “Fat Man” bomb that the U.S. dropped on Nagasaki in World War II weighed as much as two 2017 Cadillac Escalade SUVs. Since then, the weight of U.S. atomic bombs has shrunk considerably, as scientists have refined the physics of the devices and streamlined how they are armed.

With the last generation of nuclear weapons designed in the 1980s, engineers at Los Alamos National Laboratory produced the W88, weighing only 800 pounds despite having an explosive force equal to 475,000 tons of TNT — in

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