Upvote:1
In 1916 the U.S. Army ordered up some Harleys with sidecars to help track down Pancho Villa in the deserts along the Mexican border, and Bill Harley developed machine-gun mounts for the sidecars. Rider magazine
According to that article the first sidecar was invented between 1893 and 1903; 1916 seems like a plausible date for the first military motorcycle.
"plausible" just means that someone else may find an earlier date.....