He then became lieutenant from 29 December 1916 and captain on 1 August 1918, serving in the 22nd, 12th and then again in the 7th Bersaglieri Regiment; in November 1917 he was awarded the Bronze Medal of Military Valor for having led a successful counterattack during the First Battle of the Piave.

[1][2] At the proclamation of the Armistice of Cassibile, on 8 September 1943, Croce was in command of a detachment of the 3rd Bersaglieri Regiment stationed in Porto Valtravaglia, Lombardy, consisting of two battalions as well as untrained recruits of the Regia Aeronautica.

Left without orders, he witnessed the collapse of the Royal Italian Army as the Wehrmacht occupied Italy; after a few days, as his own men were starting to desert, he decided to move with his troops to the old fortifications of the Cadorna Line, on the mountains around Varese.

[3][4][5][6][7][8] The group restored the neglected fortifications of the Cadorna Line on Monte San Martino and prepared to fight the Germans and their allies of the newly established Italian Social Republic.

On 14 November, German and Fascist forces launched a heavy attack on Monte San Martino, with overwhelming numbers (up to 2,000 men, according to some sources) and air support; after losing thirty-eight men over two days of fighting and inflicting serious losses on the attackers, including a bomber shot down and some armoured cars, Croce's heavily outnumbered and outgunned partisans were forced to abandon their positions and escape towards Switzerland, marking the end of one of the first battles of the Italian Resistance.