My great grandmother' first husband went missing while serving with the army (or possibly as a mercenary?) during the period 1891 to 1895. She lived in Niedersachsen, in Bremen, and emigrated to the US in 1895 without proof of her husband's death. We don't know if he was killed, MIA or AWOL and just abandoned them.

Is there any way to know which conflict he was serving in? I can't find any wars that Germany was involved in at the time.
Thanks!