In August 1914, after the start of World War I, Ponticelli joined the 1st Régiment de Marche of the French Foreign Legion at 16 years old. He had to lie about his age as he was still to young to join the army. His older brother, Céleste Ponticelli, had also joined the regiment.Ponticelli said that France had done much for him, and being in the army was his way of showing his thanks. He fought at Soissons in Picardy, northeast France and at Douaumont, near Verdun. Ponticelli worked digging burial pits and trenches. 