My greatgrandfather was ethnic Polish but was in the Prussian army prior to 1890. How can I find his records?
There are almost no records left, the Heeresarchiv Potsdam was bombed in April 1945 and destroyed by fire. The records destroyed include also those of servicemen of WWI (please see also How to find information on German soldiers from World War I and World War II?)
Please specify your question (name, time of duty, e.g. photos), maybe secondary literature might help (but this is unlikely as well).