Since the Unification of the states of Germany in 1871? Once; World War Two.

Since the creation of Poland I counted 22 wars between Poles and Germans. It's probably not accurate. Most of them were in XI-XII century. At the beginning Poland fought vs single marches that wanted to control Poland. Later Poland and HRE became allies nad fought side by side in few wars. In XIII-XVI cent. Poland fought vs Branderburgy for western Pomerania region. In the same time, after losing few wars Teutonic Order became Prussia and was Poland's vassal, but since almost the beggining they wanted to be free and fought in many wars against their sovereign. Later Prussia started to become stronger and Polish-Lithuanian Commonwealth- weaker. That led to partitions. During the partitions there were few uprisings agains Prussian control of western Poland. Than Poland was recreated and had few small wars for borders with Germany ( 3 Silesian Uprisings, Wielkopolskie Uprising), and WWII was last one.