The Soviet Union began making nuclear weapons because of thefear that the United States might use the nuclear weapons they hadbuilt and used during WW2 on the Soviet Union.The United States began making nuclear weapons during WW2because of the fear that Nazi Germany might be making nuclearweapons and would rapidly defeat England with them, denying accessto a convenient place to mass troops for an invasion of Europe (andalso use the nuclear weapons on any invading troops). There wasalso a fear that eventually Nazi Germany might someday use nuclearweapons on the United States.Every country that has actually made nuclear weapons made thembecause of this same fear: that Another Country might have nuclearweapons and use those nuclear weapons to attack them.The only country ever actually attacked using nuclear weapons(Japan) had no idea that the United States had nuclear weapons andtheir own project doing the preliminary studies of nuclear fissionneeded to design nuclear weapons had been stalled for many yearsjust trying to build cyclotrons, due to the unavailability ofusable high frequency high power vacuum tubes during WW2 in Japan(a problem that also limited Japan's development of RADAR systems).They were so shocked by the bombings that many Japanese scientistssent to evaluate the damage did not understand what they wereseeing and the leadership of Japan initially disbelieved thereports of the scope of the damage done.