Question

what was the root cause of the containment policy?

Answer 1

Communism.

After WWII Truman realized that it would be well nigh impossible to liberate the nations in East Europe and Asia that had fallen to communist takeovers. The containment policy, aka the Truman Doctrine, was about ensuring that communism went no further and to back that up diplomatic, economic and military aid was given to those nations facing or potentially facing threats from communist neighbors or had communist backed insurgencies.