“But the overthrow of the power of the bourgeoisie and establishment of the power of the proletariat in one country does not yet mean that the complete victory of socialism has been ensured. After consolidating its power and leading the peasantry in its wake the proletariat of the victorious country can and must build a socialist society. But does this mean that it will thereby achieve the complete and final victory of socialism, i.e., does it mean that with the forces of only one country it can finally consolidate socialism and fully guarantee that country against intervention and, consequently, also against restoration? No, it does not. For this the victory of the revolution in at least several countries is needed. Therefore, the development and support of the revolution in other countries is an essential task of the victorious revolution. Therefore, the revolution which has been victorious in one country must regard itself not as a self-sufficient entity, but as an aid, as a means for hastening the victory of the proletariat in other countries.”
— Stalin, The Foundations of Leninism
The Espresso Stalinist 
A question which lately I have struggled with! Can anyone answer it?
By the end of the 2nd WW, though not in the advanced capitalist countries but atleast in East Europe & plus China SOVIET POWER had been established. Then, why ultimately by 1990s the Soviet System collapsed.
USSR plus China if not exactly half the world was still half the world in the SOCIALIST CAMP.
Logically & dialectially this means if the Soviet system collapsed it collapsed not because a ‘socialist revolution’ didn’t break out in the US or the so-called ‘advance capitalist states but the Soviet system collapsed because of the ‘internal contradictions’ inherent in the Soviet system.