WW1 wasn't fought against fascism. Italy wasn't fascist then either

From your own linked source above:

fascism, political ideology and mass movement that dominated many parts of central, southern, and eastern Europe between 1919 and 1945

Italy’s fascist movement also began after World War I, although it achieved power in the mid-1920

Fascist parties and movements came to power in several countries between 1922 and 1945

(For context, WW1 ended in 1918)

Although I'm sure many WW1 vets later became antifascists.