Answer:
Neither country wanted to engage in a nuclear war that would lead to worldwide destruction.
Explanation:
The US and USSR became arch enemies soon after World War II ended, and that continued to be the case until the USSR seized to exist. The two countries became opponents and developed very bad and tense relations because they had different ideas as to how the world should shape up, but also because both of them wanted to be the only dominant power in the world.
Both countries were heavily armed and the weaponry was constantly icnreasing, with newer and better weapons constantly been created to gain an advantage. Nuclear weapons were abundant at both as well, but they never got used in an open war between them. The reason was simple, if a direct war between them occurred, there would have been worldwide destruction and everyone will lose, so that was kept as the last option in case of an attack by the other. Not to be mistaken though, these two were constantly in war with each other, not just diplomatic, but also military one, just that they used other countries as battlefields where they supported a particular side.