Just saw ROUGH RIDERS again on Fri nightn (an awesome telemovie depicting the Rough Riders in the Spanish-american War, with Tom Berenger playin the part of TR SO well)- TR in WWI actually raised more Rough Riders for service with the British, but ended up not being sent into action...
(wiki quote)
World War I
Just after the United States entered the war against the Central Powers, the U.S. Congress gave Roosevelt the authority to raise up to four divisions similar to the Rough Riders. Roosevelt immediately selected eighteen officers (including: Seth Bullock, Frederick Russell Burnham, and James Rudolph Garfield) to raise a volunteer infantry division, and began corresponding with Newton D. Baker, Secretary of War. After several months, many more men joined Roosevelt's World War I volunteers, but Baker refused to offer any assistance or guidance to the new unit. Frustrated, Roosevelt telegrammed President Woodrow Wilson requesting his assistance; however, as Commander-in-chief, Wilson refused to make use of the volunteers and Roosevelt disbanded the unit
So, WI the div of WWI-era Rough Riders saw action in France before the formation of the AEF, with the former's combat experience being a leavening experience for when the latter came to fight ? Could these Rough Riders have been utilised by the British as shock troops in a similar way to the CEF, AIF & NZEF ?