They were both very wary of one another. The two countries ended up rising up to be some of the most powerful military powers in the world, and both followed completely different ideologies, however, as well as this, they had both a very large stockpile of nuclear weapons. A war could almost assuredly lead to the use of these weapons, which would cause MAD, or Mutually Assured Destruction, which is a catastrophic end-of-the-world scenario in which two or more great powers launch multiple nuclear weapons to destroy each other, taking down the entire world with them. This fear of MAD caused both sides to simply go to threats and undercover operations, instead of outright wars.