It depends on when you think the Cold War started. If you argue that the origins origins of the Cold War go back to the Bolshevik revolution, then the responsible party for starting would be the United States , for the reason that the United States sent 13,000 troops (known as the American Russian Expiditionary force or by its nickname , the polar bear expedition ) to the Russian cities of archangel and Vladivostok in 1918 as part of an allied force to the overthrown the new Bolshevik government.
