I didn’t realize how precarious the situation was in November of 1917. The very existence of the 42nd Division, the Rainbow, was in question. There were so many competing interests, both within the American military establishment, as shown here, and among the Allies. Pershing and Wilson were under almost constant pressure to place American soldiers in foreign units, under foreign command.
On arriving in France, however, it was other American units that wanted the men of the 42nd Division to fill the ranks. Author Donald Smythe in Pershing: General of the Armies explains (pgs. 61-62):
By November the AEF had four divisions in France: the 1st, 2d, 26th, and 42d. The first three were short 20,000 men. Since it was imperative to bring them up to full strength, it seemed logical to take the recently arrived 42d Division and break it up as replacements. General Mann, its commander, was unfit and scheduled soon to retire. It had not yet begun its training as a division, its artillery had not yet been assigned its material, and it was the only division of the four with complete personnel ready to feed into needy units. Accordingly, Fox Conner, Acting Chief of Operations, supported by Harbord, recommended classifying the 42d as a replacement division.
The only trouble was, the 42d Division was Secretary Baker’s personal creation. The previous summer he had remarked to Douglas MacArthur, who was on the War Department General Staff, that he “wished we had a division in which there were components from every State so that each State could take pride in the fact that some of its own boys were among the first to go.” MacArthur suggested that many National Guard divisions had surplus units which might be joined together to form such a unit. … Thus was born the famous Rainbow Division. Maj. Gen. William A. Mann, Chief of the Militia Bureau, was enthusiastic about the project and became the division’s first commander. MacArthur became its Chief of Staff.
Arriving in France, however, the Rainbow found itself being cannibalized. Equipment, supplies, and clothing were taken from it to supply deficiencies in the 1st and 26th Divisions. Thirty-three of its finest officers, including the incomparable Summerall, destined to become a postwar Army Chief of Staff, were reassigned either to Chaumont or to other divisions. Mann and MacArthur protested vigorously, the latter going so far as to leak to newsman Herbert Corey as story which Chaumont subsequently killed.
While far from a vigorous general, Mann was a very active politician with many influential friends in Washington because of his years there as a Chief of the Militia Bureau. MacArthur had the respect of Secretary Baker and the ear of the press. The division itself, representing units from twenty-six different states and the District of Columbia, had received extensive publicity and acquired a widespread constituency concerned about its survival…
To Chaumont, however, the 42d Division was a test case. By all the rules of logic, it should be the replacement division. If it was not, it would establish the precedent of allowing National Guard units special consideration, a policy fraught with danger. If National Guard units could not be used for replacements, then National Army units (those formed by the draft) must be. Since these were not expected in Europe for some time (the first did not arrive until April), AEF divisions would continue to be understaffed and underequipped. Fox Conner told Harbord that if they allowed the Rainbow to get away with their political maneuverings, it would become increasingly difficult to control National Guard divisions. There would not be one U.S. Army in Europe, but two.
The Rainbow was indeed maneuvering. Mann, MacArthur, and others alerted influential friends back home, and before long the War Department received a barrage of letters and telegrams demanding that the division be saved. In Europe MacArthur collared Harbord, an old friend from Philippine days, and asked him personally to inspect the division and judge “whether such a splendid unit should be relegated to a replacement status.”
Harbord did and was impressed. He then drew up a list of reasons for Pershing, pro and con, about making the 42d a replacement unit. They were mostly con. Perhaps his most cogent reason was the last: “I much fear that if you used it for replacement without notice to the War Department that you will not be permitted on the other hand if you ask the War Department that you will not be permitted to do it.” That settled the matter. Pershing designated the next division to arrive, the 41st (also National Guard troops) as the replacement unit. The Rainbow Division was saved. It became one of the best divisions in the AEF.
Note that Chaumont, a community in France, was the site of the General Headquarters, American Expeditionary Force, in World War I. Unfortunately Smythe makes it sound as if Chaumont is a person, an actual general. The National Guard comprised 40 percent of the U.S. combat divisions in France during the war.