Truly one of the most mind-bogglingly surreal movies I've ever seen, teaching kids that all they need to do to survive a nuclear war is duck down and cover their heads; and quite rightly satirised in the 'South Park' TV show. Watching it today it's hard to imagine that the US government could have made it as a serious training film, or that American teachers could have shown it to their kids with a straight face.
More seriously, I imagine that when they first began planning the movie they were probably still in the A-bomb era where it might have made a difference: the damage from such a bomb was fairly localised so a single bomb in a major city would kill only a small fraction of the population... but by the time it was being shown to kids H-bombs could obliterate an entire city in one go, and 'Duck and Cover' tactics were utterly pointless.
More seriously, I imagine that when they first began planning the movie they were probably still in the A-bomb era where it might have made a difference: the damage from such a bomb was fairly localised so a single bomb in a major city would kill only a small fraction of the population... but by the time it was being shown to kids H-bombs could obliterate an entire city in one go, and 'Duck and Cover' tactics were utterly pointless.