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. Facing an enemy who was determined to hold its positions, these National Guardsmen fought with courage and determination to gain what was often only yards of ground, and did so at a deadly cost. In Nine Desperate Days: America's Rainbow Division in the Aisne-Marne Offensive, historian Robert Thompson chronicles the hardships and tenacity of the men from the 42nd Division during this pivotal campaign. The Americans did not break despite heavy losses, and were able to drive the Germans back from territory they initially gained. The efforts of the Rainbow Division during Aisne-Marne were key to the ultimate Allied victory and are a symbol of American valor and sacrifice during the "war to end all wars."