For a to be said a cause of b, there must be, between a and b, a constant relationship having some proprieties such as, among others, the asymmetric propriety: a precedes b and it never happens that b goes before a. Lets say that R is the reason of a, when R is a concept under which a falls. So, for instance, certain facts concerning the dispute between the King of England and the King of France are the cause of Hundred Years War. The reason of Hundred Years War consists in its falling under the concept: "the reinforcement of national monarchies".
When one mistakes reason with cause, one is also apt to attribute to the former the same kind of existence which belongs to the latter (note that the word reason is so dangerously close to the words motive, intention), as if a Rational Will was actually driving the world according to a plan and could really yield, one by one, the whole series of events which history is made of.
Am I wrong, if I say that Marxism has often (and largely) nourished itself with this mistake ?
Moreover: wouldnt it be odd, for such a Marxism, to consider its XX century history as if it was just a question of causes (of concrete historical circumstances ) ;as if these causes didnt involve the concept under which all of them fall (their eidos), i.e. the same concept of communism ?