On 27 October 1917 he was captured near San Pietro al Natisone during the battle of Caporetto, being then held as a prisoner of war in Grahovo, later in Rastatt and finally in Celle, where he remained until December 1918.

An ardent nationalist since before the war, he immediately joined the nascent Fascist movement, becoming a squadrista; he participated in the March on Rome and in February 1923 he enlisted as seniore (major) in the newly formed Voluntary Militia for National Security (MVSN), first commanding the "Cacciatori del Tevere" Legion, and then an autonomous Blackshirt unit tasked with guarding the confinement colony in Lipari.

[1][5] In July 1941 he was appointed commander of the 63rd Assault Blackshirt Legion "Tagliamento", part of the Italian Expeditionary Corps in Russia (CSIR), participating in the Axis advance into the Soviet Union, in the region between the Dnieper and the Don; in October 1941 he was awarded a Silver Medal of Military Valor for his role in the capture of Pavlohrad, and two months later he earned a silver medal for having repulsed a Soviet offensive in the so-called "Battle of Christmas".

[1] At the proclamation of the Armistice of Cassibile, on 8 September 1943, Nicchiarelli was in Ljubljana, at the command of a blackshirt unit; he decided to continue the war alongside Germany, joining the Italian Social Republic when it was established later that month.

Another rival of him was Marshal of Italy Rodolfo Graziani, an enmity which dated back to the time when Nicchiarelli had been his subordinate in Africa at the start of the war.

In April 1945, as the Italian Social Republic collapsed, he opposed the idea, suggested by some of the most hardliner Fascist leaders, of making a final stand in the Valtellina Redoubt.