Alsace-Lorraine, for most of the late 19th and early 20th centuries, was always a region of conflicting interests between two specific states: France and Germany. Neither state wanted the other to have any semblance of control over the area.
Thus, the challenge here is to make Alsace-Lorraine a compromise area, making it either an independent, neutral buffer state or a jointly administered region between Germany and France.
Time frames don't matter unless the PoD is waaaay back.