_____ is preventing the Soviet Union from expanding its influence around the world.

I assume that we are meant to fill the blank in this question with a term that means something like “preventing the spread of Soviet influence.”  In other words, I will provide a term that means this, not a factor that prevented the Soviet Union from spreading its influence.

If this is correct, the best answer to your question is “containment.”  This was the main foreign policy that the United States pursued for most of the Cold War.  This was a policy that said that the US would try to prevent communism (and Soviet influence) from spreading out any farther than it already had.

The United States essentially realized that there was no way that it could use its military to roll back communism.  It was not going to invade Poland or the USSR.  Therefore, the only choice the US really had was to try to prevent communism from spreading any further.  If the US could keep communism contained, the rest of the world could live in freedom. 

The word you are looking for to fill this blank, then, is “containment.”