Answer: The Soviet Union had shown that it wanted to expand its area of control in Eastern Europe.
Additional detail regarding NATO:
The United States, along with Canada, joined with ten European countries in signing the North Atlantic Treaty in 1949. This created the North Atlantic Treaty Organization (NATO), which was a defensive military alliance of democratic states over against the expanding threat of communism felt in the Cold War environment.
The ten original Western European members of NATO were the United Kingdom, France, Belgium, the Netherlands, Denmark, Norway, Portugal, Italy, Iceland, and Luxembourg.
