He finished third grade twice in order to improve his basic education and in 1908 started to work as an apprentice blacksmith under his older brother Giuseppe in his workshop. In 1911 his father died in a tragic accident – a flashflood that overran his electrician workshop that had been built on the bank of the Troggia Waterfall. Tantardini later recalled that his father's bones were not found until 1914. In 1915 he began work at the Ansaldo Power Plant in Genova. He enlisted in the Italian armed forces after World War I broke out in Europe and was exempted from active service since he worked in a corporation that was war-supporting and contributed to the Italian war effort. The Italian defeat at Caporetto on 24 October 1917 saw him recalled and after two months of training he was sent to the front lines in January 1918. But just 48 hours later the German soldiers captured him and 60 others and led them to Vittorio Veneto where he was assigned to work on the railroads. He suffered from hunger and the cold weather that affected his health. He was relocated often from one concentration camp to another until he managed to escape his captors; he was sent to Udine and Gorizia before being sent to Belgrade in Serbia. During his time as a prisoner of war he slept on the ground and had two meals consisting of beetroots cooked in water, with transgressions prompting a whipping for prisoners who angered the guards. He planned his escape in December 1918 alongside four others, crawling through a drainage channel before reaching Greece and then returning home. In June 1919 he arrived home "via" a ship to Taranto and a train ride to Lecco. He spent just over two weeks recovering before being sent for three months to take the Greek island of Kalymnos .