After the war was over, tensions began to grow between the US and Russia. (During the war, the US used the first nuke at Hiroshima) The tension between the two countries is what caused there to be the cold war after WWII. The US and Russia were both "fighting" to get ahead with technology, but both countries were afraid to use it because of how much worldwide devastation it would cause. (I'm not sure if this directly explains the question, but technically this was a result of WWII)
