The 42nd Infantry (Rainbow) Division's history began with America's entry into World War I. Amidst the rush by America to mobilize, individual states competed with each other for the honor to be the first to send their National Guard units to fight in the trenches of Europe. To check the negative implications of this competition and to minimize the impact the mobilization could have upon any one state, the government decided to create a division composed of handpicked National Guard units from 26 states and the District of Columbia. As a result of this unified effort, the 42nd Infantry Division was born on August 5 and organized in September 1917 at Camp Mills on Long Island, New York.

Colonel Douglas MacArthur, who had been instrumental in the forming of the Division, said shortly after its completion. "The 42nd Division stretches like a Rainbow from one end of America to the other."

The division started sailing from New York in October and completed the Atlantic crossing in November and December 1917. It trained in Eastern France until mid-February when it was partnered with the French at Baccarat, a city in Lorraine.

The Rainbow was then prominently placed in the successful Aisne-Marne offensive fought in late July and early August at Croix Rouge Farm and on the Ourcq River. It was part of the American I Corps under Major General Hunter Liggett.

The division went on the offensive at St. Mihiel on September 11, 1918. It was one of 13 divisions in the 1st US Army for its first all American operation and the biggest American operation of the war up to then.

Immediately after helping defeat the outnumbered Germans at St. Mihiel, the Rainbow moved 60 miles to the Argonne Forest in October. In a much tougher war, it replaced the US 1st Division on October 11, 1918.

With regiments abreast, the Rainbow Division attacked on October 14 and inched back and forth in an attempt to penetrate the Hindenburg Line at the Kriemhilde Stellung, one of four German defensive lines. On October 16, at a hill called Côte de Châtillon, they successfully assaulted the most formidable part of an in-depth network of wire and carefully prepared German defenses.

Participating in the Sedan fighting in November, the “Rainbow” was on the “Drive to the Rhine” when the war ended on the 11th. The 42nd Division had remained in almost constant contact with the enemy for 174 days.

During its time in France, the 42nd Division incurred one-out-of-sixteen casualties suffered by the American Army during the war. The division saw more days of combat than any other American division during the Great War and suffered a total of 14,683 casualties.

The 42nd Division's service officially came to an end after occupation duties along the Rhine in the spring of 1919, when it sailed for home from Brest, France on three battleships, arrived back in New York on April 25, 1919.