Air assaults on Saturday killed at least four people. Kharkiv’s mayor described an overnight strike as “the most powerful attack” since the beginning of the war.

KYIV, Ukraine — The covert operation was described as one for the “history books” by Ukraine’s president. In the span of a few hours on Sunday, nearly a third of Moscow’s strategic bomber fleet was destroyed or damaged with cheaply made drones sneaked into Russian territory, according to Ukrainian officials.

Could Ukraine produce a nuclear bomb and transborder missiles to halt Russia’s invasion? A global expert on atomic arms war-games this puzzle and its consequences.