Marziali volunteered in the Royal Italian Army during the First World War, fighting on the Karst Plateau as an infantry lieutenant and being seriously wounded on the Karst on 21 November 1916 and then again in Kostanjevica na Krasu on 6 August 1917, during the Tenth Battle of the Isonzo, which left him with a permanent disability.

After returning to civilian life he graduated in Law at the University of Siena in 1921, and in the same year he married the Florentine Marta Jenna, of Jewish origin.

On 1 October 1920 he joined the Florence section of the Italian Fasces of Combat, participating in several squadrist raids in Tuscany.

After the transformation of the Fasces of Combat into the National Fascist Party he was secretary of its Florence section from September to December 1922, participating in the March on Rome, and then provincial councilor from 1923 to 1926.

His wife, who had taken refuge in Badia Fiesolana, was captured and shot by the Germans, while Marziali was taken prisoner by the Allies and tried in Rome, where however he was absolved.