So, the POD has to be after the publication of at least the first issue of Das Kapital. What changes can be made to have Socialism and or Communism as the single dominant ideology on the planet. That is the only stipulation.
There doesn't need to be a single global "Soviet", some nations can be very open free and democratic, others totalitarian and oppressive. Different varieties of the ideology can be everywhere, so long as the main base of political, economic and social thought is rooted in Communism.