Hey there!
It's argued that it's the fault of the Soviet Union, led by Stalin.
This was because Stalin was greatly expanding his army and military capability. He was becoming more and more powerful, and with the addition of nuclear weapons as a threat, the Cold War began.
Remember-- the Cold War was a war of immense tension, but involved no fighting. Both leaders knew better than to start fights when two of the largest superpowers in the world were armed with weapons of destruction.
The response of the United States was nothing but defense, as usually all foreign affairs in wars we were/are involved in for the preservation of democracy, and usually that only. Therefore, the US wasn't to blame.
Hope this helps!