As North Korea dug tunnels at its nuclear test site last fall, watched by American spy satellites, the Obama administration was preparing a test of its own in the Nevada desert.

A fighter jet took off with a mock version of the nation’s first precision-guided atom bomb. Adapted from an older weapon, it was designed with problems like North Korea in mind: Its computer brain and four maneuverable fins let it zero in on deeply buried targets like testing tunnels and weapon sites. And its yield, the bomb’s explosive force, can be dialed up or down depending on the target, to minimize collateral damage.

In short, while the North Koreans have been thinking big — claiming to have built a hydrogen bomb, a boast that experts dismiss as wildly exaggerated — the Energy Department and the Pentagon have been readying a line of weapons that head in the opposite direction.

The build-it-smaller approach has set off a philosophical clash among those in Washington who think about the unthinkable.