Why did the united states assume global responsibility for containing communism?

The US assumed global responsibility for containing communism because they emerged as a global superpower after World War II.

After World War II, many countries in Europe were ruined due to the amount of fighting that took place in their countries during the war. The US, however, did not have any of the fighting take place in the continental US. This lack of damage and a strong economy allowed the US to help other countries during this time.