>>18068710 (OP)
My Scandinavian born great-grandfather was in the cavalry in France.
I would imagine it is this unit though I am uncertain
>World War I
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When the United States entered World War I, the regiment sailed for France as one of the four mounted regiments on duty with the Allied Expeditionary Force. The fighting had bogged down into trench warfare and the role of horse cavalry was nearly over. The 15th subsequently dismounted and relieved exhausted infantry units in the trenches. It was the tank that finally broke the trench lines to end both the war and signal the twilight of the horse soldier. The 15th served occupation duty after the war, and arrived at the port of New York on 19 June 1919 on the USS Panaman. It was transferred the same day to Camp Mills, New York, where emergency period personnel were discharged from the service.
https://en.wikipedia.org/wiki/15th_Cavalry_Regiment