After promotion to lieutenant, in June 1918 he distinguished himself in the Second Battle of the Piave River, earning a Bronze Medal of Military Valor.

In this capacity, during the late 1930s and early 1940s he gathered information about the political and social situation in the capital, which he passed on to his superior Riccardo Moizo, who in turn related to the king and government.

[1][2][5][4] Less than a month later, on 23 August 1943, Frignani was ordered to arrest Fascist leader Ettore Muti, suspected of being preparing an insurrection to return Mussolini to power, at his villa in Fregene.

Some historians believe that Muti was deliberately executed on the orders of the new head of government, Marshal of Italy Pietro Badoglio; others have pointed to a possible involvement of Frignani's brother Giuseppe, member of the Italian Parliament, former State Undersecretary for Finance and former Federal Secretary of the Fascist Party for Ravenna, and longtime enemy of Muti (the two had been bitter rivals for the leadership of the Ravenna section of the National Fascist Party in the 1920s).

On 23 January 1944 he was arrested by the German police, together with Major Ugo de Carolis and Captain Raffaele Aversa, and imprisoned in the SS prison in via Tasso.