The motive for the nuclear arms race between the US and the Soviet Union was a desire for power and security. This was going on during the Cold War. Each side in the Cold War thought the other side might well attack. In order to be able to defend themselves, each side needed nuclear weapons that were as good as what the other side had. Each side also wanted to pull ahead of the other side so it could be more powerful. For this reason, the two sides kept trying to build more and better nuclear weapons during the time of the arms race.