The presence of the soviet army in eastern europe at the end of world war ii ensured that would be established in poland, romania, bulgaria, hungary, and czechoslovakia.

The correct answer for the blank is "communism."

After World War II, different parts of Europe were split up into spheres of influence. For example Berlin, the capital of Germany, was split up into two different parts. The US, Great Britain, and France helped to control West Berlin and the Soviet Union controlled East Berlin.

Another example would be the countries that used to belong to the Nazi German reign. Countries like the ones mentioned in the statement, came under the control of the Soviet Union due to their close geographic location to Russia. From there, the Soviet Union set up communists systems in these countries so that they followed the same ideas/beliefs as the Soviet Union.

  the domino theory states that  if one country in a region came under the influence of communism, then the surrounding countries would follow in a  domino  effect.