The ideological differences between the Soviet Union and the US were undoubtedly a key factor in the early development of the Cold War. As stated in Source X, the contrasting beliefs of capitalism and communism meant that cooperation was very difficult. However, the attitude of Stalin mentioned in Source Y and Z, and Soviet aggression were also very significantly in the increasing of tensions during 1945 to 1953. Nevertheless, fundamentally the development of the Cold War is best explained by mistrust and misunderstanding that existed between the superpowers.