Boyaux (litterally “guts”) were communication trenches, used by soldiers to go from the rear lines to the front. This picture shows Italian soldiers going to the front line somewhere in France. Its zigzag shape was a way to protect soldiers in case of shelling or machine-gunning.
This medal was created in 1852 by Napoléon III. Starting from 1870 and until 1940, the Imperial eagle and the head of the Emperor are replaced by more republican symbols, with the 1870 date.
During and after the Great War, 1,400,000 Médailles Militaire were awarded, most of them posthumously. 185,000 medal were awarded during the conflict; 58,000 after the end of the conflict. In 1923, 1,000,000 ca. had been posthumously awarded and 320,255 living veterans had this medal. I’ve published a post a long time ago about the papers linked to this medal.