Both, but The Soviet Union was primarily responsible for starting the Cold War. The Soviet Union is responsible because they wanted the fruits of war. The US was only trying to help people and not start a war, even though the Soviet Union didn't want a war, they needed it, there for they needed the war. According to document c it states " I do not believe that the Soviet Russia declares war. What they desire is the fruits of war the indefitne expansion of their power and doctrines "