In 1919, convinced by the then captain Cesare Maria De Vecchi,[2] he joined the fascist squads, of which he quickly became a leading exponent in Piedmont, also founding a newspaper, Il Trincerista.
[3] In the meantime, having moved to Mortara, the political and economic center of Lomellina at the time, he put himself at the head of an authentic personal army, made up of hundreds of squadristi, mostly veterans.
[5] Very popular among the squadristi, after the March on Rome and the seizure of power, he was increasingly considered as an inconvenient character for his intransigent fascism and then as a real dissident, for his violent attacks against the new arrivals in the ruling class of the party, without a past of convinced militancy in the movement.
He challenged the then political secretary Francesco Giunta, a favorite of Mussolini, to a saber duel which took place in April 1923 in Rome and was suspended by the referees when both duelists were wounded.
In December 1923 the disciplinary council of the PNF suspended him from all the positions he held, the Pavia federation was placed under police administration and in February 1924 he was expelled from the National Fascist Party.
[6] On 12 March 1924 Forni was attacked at the central station of Milan by some squadristi including Dumini, Volpi, Malachia, the consortium members in the so-called Fascist Ceka, the same ones who would shortly thereafter kill Giacomo Matteotti.