As it's Armistice/Veterans'/Remembrance Day weekend: my grandfathers, both of whom served with the US Army forces in France, in 1918.
My mom's father left his farm at age 32 and was a cook/baker for an enlisted unit; family legend is that his mess was so popular that officers often dined with the troops to get a better meal. My dad's father, in his mid-20s at the time, was a chemist by education and trade, and served in a medical unit closer to the front lines. As a child, I was fascinated with his doughboy helmet and gas mask, which he kept on the garage wall. Neither talked much about the war, but rather the duty they felt to serve the country that had welcomed them as immigrants. I think of them often, particularly this time of year.