What were the main domestic and international factors leading to the post war red scare?

If "post war red scare" is referring to "post WW2...", there was no "Main...factor(s)..." leading to the "red scare." There was only ONE factor: Nuclear weapons. Ghengis Khan never had those weapons, Napoleon never had those weapons, Rome never had those weapons...and neither did Adolph Hitler or Imperial Japan! For the first time in the history of the human race, we now had a weapon(s) that could extinguish mankind. This wasn't some super battleship, or complicated chemical warfare weapon, or even some new super bomber like the B-52 Stratofortess...this was "The Bomb!" And the "Red's" had it too! That was the RED SCARE.