Both were to blame.

If its for your essay i recommend if its an in-class essay to have 4 main points.

Paragraph 1 = Capitalism and its incompatibility economically and socially with socialism

Paragraph 2 = Anti-Communist sentiment in the USA i.e. the Red Scare in the 1920s and reference the Long telegram by secretary of state to the president after WW2 (forget who wrote what)

Paragraph 3 = Detail Russia's state after the war i.e. losing 40 million men and women, infrastructure demolished, cities in rubble and a population that had endured in comparison to the figurehead of capitalism America. Detail the Long Telegrams Russian Equivalent (written in 1947 i believe).

Paragraph 4: Detail the reaction to communism by the allies directly after WW2 i.e. Churchill's push to invade Russia and the demon-ization of the Russian by western media. Try to find Russian equivalents.

After that write out the paragraphs and see if you can personally decide if one side is to blame and change the essay accordingly.

You cant say just 1 side was to blame based on what some people on the internet tell you, do some research and decide for yourself (thats the point of a take home essay).