American forces entered World War I combat in October 1917, but it was not until July 1918 that they went on the offensive for the first time. Among the units selected for this operation was the 42nd Division, or the "Rainbow Division" as it was known popularly. This division, which was composed of National Guard units from twenty-six states, including Joyce Kilmer's 69th New York Infantry, would spend 164 days in combat, a number exceeded by only two other American divisions. However, it was the nine days from July 25 to August 2, 1918 that were the most terrible and heroic in the division's history. General of the Army Douglas MacArthur, who served as the division's chief of staff, later said of the Rainbow Division's fighting during those pivotal days, "There was neither rest nor mercy." During those nine days, the Rainbow Division would spearhead the Allied attack in the Aisne-Marne region, the final phase of the Second Battle of the Marne, which saw the last major German offensive on the Western Front defeated. Unfortunately, American leadership proved to be completely unprepared for a dynamic war of maneuver. Basing decisions on wildly inaccurate information about the Germans, senior commanders ordered the men of the Rainbow Division to make ill-advised assaults again and again with little support. Facing an enemy who was determined to hold itspositions, these National Guardsmen fought with courage and determination to gain what was often only yards of ground, and did so at a deadly cost.