France and Germany marshaled 3.7 million soldiers for the Western offensives that began World War I in August 1914—with Britain adding an additional 130,000. In the decisive days between Sept. 5 and Sept. 11, the two sides threw two million men into desperate combat along the Marne River, the right tributary of Paris's famed Seine. More than 610,000 men were killed and wounded during the month-long campaign—two-thirds the number of casualties suffered by the U.S. in the whole of World War II. But such numbers do little to bring home the ordeal. To reach the Marne, Alexander von Kluck's First...