One hundred years ago, Allied troops readied themselves for a forgotten battle that turned the tide of World War I. Thousands of American servicemen marched through the night to position themselves for a surprise strike against the Germans. A thunderstorm raged the third week of July 1918. Other than the occasional arcs of white-hot lightning, the night was so dark that the American Marines marched with each man holding one hand on the pack of the man in front of him to avoid getting lost. They hadn’t eaten in more than a day, and the men were bone-weary. Still, they...