Hello ,a comment . At the outbreak of ww1 many foreigners resident or not in France volunteered to service in the French Army.this corp of volunteers included citizens of USA . The Swiss writer Blaise Cendrars was among the volunteers . he wrote a book The chopped hand , in which related his war experiences . Two points of his book i think are in connection with the theme . One that all the volunteers were officially incorporated into the Foreign Legion . as infantrymen . Two ,during 1915 ,the French High Command searched the files looking for volunteers to serve in the heavy Artillery . the neccesity of artillerymen endured all the duration of the war . so if Dr Brodeur enlisted in the Foreign Legion it was because this Corp was the only allowed to receive foreign volunteers . acting as a sort of Depot . and being a graduate its not surprise that he was oriented to the Artillery,