The 42nd Infantry Division (42ID) ("Rainbow") is a division of the United States Army National Guard. It was nicknamed the Rainbow Division because, during rapid mobilization for service in WW1, it was formed from 27 National Guard units from across the US. The division was engaged in four major operations between July 1918 and the armistice in November 1918, and demobilized in 1919. Since World War I, the 42nd Infantry Division has served in World War II and the Global War on Terrorism (GWOT).

The American Expeditionary Forces (AEF) was a formation of the United States Armed Forces on the Western Front during World War I, comprised mostly of units from the U.S. Army. The AEF was established on July 5, 1917, in France under the command of then-Major General John J. Pershing. It fought alongside French Army, British Army, Canadian Army, British Indian Army, New Zealand Army and Australian Army units against the Imperial German Army. A small number of AEF troops also fought alongside Italian Army units in 1918 against the Austro-Hungarian Army. The AEF helped the French Army on the Western Front during the Aisne Offensive in the summer of 1918, and fought its major actions in the Battle of Saint-Mihiel and the Meuse-Argonne Offensive in the latter part of 1918.