newsreel narrator: president truman's dramatic announcement that russia has created an atomic explosion sends reporters racing for flushing meadow, where russia's vyshinsky arrives to address the united nations... this puts the u.s. in a really complicated position, because it's no longer the only country with nuclear weapons. suddenly, you have the possibility that if war with nuclear weapon broke out between two states that had a fair number of them, they could, in a matter of hours, destroy themselves. (civil defense siren blaring) newsreel narrator: we must all get ready now, so we know how to save ourselve if the atomic bomb ever explodes near us. and one of the possible options scientists and policy people lobbied for was to build the hydrogen bomb as the sort of next step. (explosion rumbling) snowden: the hiroshima and nagasaki bombs were on the order of 15 kilotons of tnt,