When Defense Secretary Mark Esper visited the headquarters of the U.S. Strategic Command in Omaha in late February, he was given a tabletop demonstration of a hypothetical lose-lose confrontation with Russia. It begins with Moscow using a “battlefield” nuclear weapon against a target in Europe and leaves the United States with little choice but to respond with a low-yield nuclear weapon of its own.

A Pentagon briefer who described the exercise to reporters didn’t disclose how the imaginary war game played out, except to confirm it went nuclear — and in a world where the primary purpose of nuclear weapons is deterrence, that counts as a loss.