Value-form
The Western Left academics are nowadays very happy to talk about "varieties of capitalism", but talking about "varieties of socialism" remains almost an absolute "no-no". Most Marxists continue to believe in "only one" (Marxian) socialism: either that, or nothing at all. "Either" a society or movement conforms to the true concept of socialism, "or" it does not; and if it does not, it "cannot be" socialism. However, precisely "because" there are many different Marxist beliefs about socialism, Marxists cannot agree among themselves whether Soviet-type societies were socialist, capitalist, or something else. Some countries meet with the moral expectations which Marxists have, and others do not.Originally Marx and Engels themselves had heckled the advocates of "true socialism", but later Friedrich Engels and Karl Kautsky promoted Marxist socialism as the one and true "scientific socialism" within the Socialist International. When he was 43 years old, Lenin stated: "The Marxist doctrine is omnipotent because it is true". Lenin's Communist International (a deliberate split with the social democrats) adopted Marxist socialism as its official political doctrine.
Marxist schools of thought
First, Lenin's followers generally view his additions to the body of Marxism as the practical corollary to Marx's original theoretical contributions of the 19th century—insofar as they apply under the conditions of advanced capitalism that they found themselves working in. Lenin called this time-frame the era of imperialism. For example, Joseph Stalin wrote: 
The most important consequence of a Leninist-style theory of imperialism is the strategic need for workers in the industrialized countries to bloc or ally with the oppressed nations contained within their respective countries' colonies abroad in order to overthrow capitalism. This is the source of the slogan, which shows the Leninist conception that not only the proletariat—as is traditional to Marxism—are the sole revolutionary force, but all oppressed people: "Workers and Oppressed Peoples of the World, Unite!".