The Greater Poland uprising of 1918-1919, or Wielkopolska uprising of 1918-1919  or Posnanian War was a military insurrection of Poles in the Greater Poland region  against German rule. The uprising had a significant effect on the Treaty of Versailles, which granted a reconstituted Second Polish Republic the area won by the Polish insurrectionists. The region was part of the Polish-Lithuanian Commonwealth before the Second Partition of Poland in 1793 when it was taken over by the German Kingdom of Prussia.
Question: What was Poland fighting?
Answer: German rule