The American Legion was founded in 1919 as a veterans service organization for soldiers who served in World War One, and they chose the 10th anniversary of America’s entrance into the war to make a mass pilgrimage back to France. Calling themselves the Second American Expeditionary Force, some 20,000 Legionnaires traveled to Paris in September 1927 for the Legion’s 8th annual conference. Far more than just formal organization business, it was a chance for veterans to see the battlefields again, meet with their French counterparts, and generally have a really good time. The event was coordinated years in advance, and one small element of it was the minting of commemorative medallions – aka challenge coins. This example was sent to me by a viewer, who got it from the widow of an American WWI veteran who attended the event.
The American Legion has two very cool pieces or history relating to the 1927 Convention in their digital archive. One is a compilation of 16mm film taken of the event, including festivities on Coney Island prior to embarkation and footage of the transit over. You can see that here:
They have also digitized the original program for the event, which includes a lot of information about the battlefield tourism available and general advice for Americans overseas. You can download it (26MB PDF) here: