How were Fascist dictators able to rise to power in Italy, Germany, and the Soviet Union following WWI and helping to cause WWII?
First of all, let us note that Stalin is not considered to have been a fascist dictator. He was a communist dictator. However, the same general sorts of factors led to his rise to power as well.
In all of these cases, dictators were able to rise because of chaos and unrest of one sort or another. In the USSR and Germany, there was unrest because of the poor performances put on by the previous governments during the war. In all three countries, there was unrest because of economic conditions. In Russia, this had to do with the general backwardness of the economy and the oppression of the poor. In Germany, it was because of war reparations. In Italy, it was because of worker strikes.
In all cases, then, the dictators were able to rise because of chaotic times in their countries.