The Paris Treaties and aftermath definitely had a major impact on fascism's rise to prominence, but you have to remember that the ideology existed in a sort of embryonic form before the war even broke out. You can read my post here where I describe the genesis of fascism as a distinct political ideology to get a better understanding of how and why it was appealing in the early 20th century. A crucial element that I regrettably did not focus on was how fascism’s rise was also largely the result of decades-old class conflict in Italy and Germany.

Fascism also emerged in countries like Italy and Germany because it was a brutal tool used to suppress the organized working class and destroy socialist and communist movements. There had been divisions between the ruling elites, lower-middle class and mass base (workers and peasants) since the 19th century. Economic and social elites in these countries perceived fascism as a weapon that they could use to crush workers' rights which threatened their hierarchy and private property, and to undo the 'diktat' of the Paris Treaties which hindered their capacity to profit from military production or from gaining further territory. They didn’t necessarily believe in the tenets of fascism, but they thought that a violent, reactionary mass movement could be used as a weapon and controlled. In both Italy and Germany, fascists were brought to government by conservative elites for these reasons.

Wherever fascism reared its head, the violent suppression of the left and mass base followed. In Italy, workers had occupied factories and socialist peasant leagues appropriated the estates of landlords in the chaos of the months following WWI. Gangs of fascist blackshirts were hired by factory owners and landlords to brutally supress these groups; blackshirts would roll into “socialist” villages on trucks provided by landlords and run around beating any peasant suspected of being a socialist, or forcing them to drink castor oil. The Nazi SA was used in a similar capacity against communists and trade unionists, but that ideology’s focus on anti-Semitism meant that they also focused on violent attacks against Jews and their property. Fascist brutality against workers was even more barbaric during the Spanish Civil War. Franco used terror and violence to totally destroy the will of the working class to undermine the republican capacity to fight back which not even the Nazis had accomplished; Spanish fascists viewed workers as contaminated by Bolshevism which had to be erradicated. When they took Seville, Franco's forces launched a wave of executions that virtually destroyed the working class population. Mass executions were carried out in working class neighborhoods, and the bodies were left in the streets for hours as an example to the other workers. Fascism may have developed independently, but it only emerged out of obscurity through the actions of conservative landowners and political elites who wanted to destroy the organized left and reorganize Europe’s political landscape.

The lower-middle class also had a reason to support the destruction of the organized working class. Through organized labour, the working class had gained wages that approximated those of the better educated lower-middle class. They also wanted to roll back the gains that the working class had achieved through peaceful labour organizing because it threatened their status in the social and economic hierarchies. Consequently, members of the petty-bourgeoisie formed the backbone of fascism’s initial mass movement, which was strengthened by veterans of the First World War who made up its street gangs.

Crises in immediate post-war Italy and 1930s Germany are what made those conservative elites bring fascists to power. In both countries, they wanted to create a stable parliamentary majority to govern effectively and thought that this could be achieved by bringing popular fascists into their governments. The conservative political class had a general attitude that fascism was an inevitable but temporary remedy for a parliamentary system that had gone wrong through the influence of the working class. Hitler managed to outmaneuver those conservatives and establish a personal dictatorship, however in Italy, Mussolini ruled in cooperation with conservatives throughout the war, until his own party deposed him in 1943.