Initially I wrote something longer and much more complex, but I will keep this short and simple.
According to the official Marxist-Leninist ideology in Albania, the purpose of the communist movement was “leading it (the country) from its backward semi-feudal state to socialism, bypassing the phase of advanced capitalism” -History of The Party of Labor of Albania, 1st edition (p.6-7).
Indeed this was the ideology of practically every Marxist-Leninist state of the 20th Century. But what did Marx believe? Precisely the opposite, that socialism would likely only be achieved by building off of the progress made by the advanced capitalist phase of development, in the most advanced countries first as a result of the internal contradictions of the capitalist system and not the external contradictions of imperialism.
It must be said that Marx did have high hopes that Russia in particular could go through a new phase of development, bypassing capitalism. But he never based such a view on any evidence, purely speculation, as he stressed. All of Marx’s works support the notion that socialism can only be achieved in the most advanced capitalist countries first. This view is supported by the overwhelming majority of Marx’s writings on capitalism.
Thus the failure of the 20th Century was not by any means a failure of ‘socialism’, but of the attempt to ‘skip over’ an advanced capitalist stage of development, to reach socialism in semi-feudal countries without the help of revolution in advanced capitalist countries. Thus we go back to the theory of ‘socialism in one country’, and its failure.