The success of the Russian Revolution led to hope that other European states would follow, specially Germany, which was ripe. Instead, through violent right wing reaction, and the SPD betraying socialists and the cause, the Weimar republic ended up conceding to the Fascists, brought siege socialism to USSR and allowed the US to entrench itself as a global superpower. It is basically the idea of a hinge point where an alternate, communist reality, would have been the logical conclusion.