Answer:
The fact that there would be large scale destruction and devastation throughout the world.
Step-by-step explanation:
Though both the United States and the Soviet Union acquired powerful nuclear weapons during the Cold War, it was less likely that the two countries would ever use them.
The fact that nuclear weapons would wipe the whole existence on earth and would bring massive destruction was always feared by both the countries. It was a well established fact that full-scale nuclear war would be the end of the human race.
Also, the second strike capability and mutually assured destruction would undoubtedly wipe both the two countries if the war ever occur. Therefore, both US and the Soviet Union did not use any nuclear weapon against each other.