What war did Anne Frank's father serve in and why was this ironic?

As a German, Otto Frank's father served in the German army in World War 1 (1914-1918) - and like all German Jews was persecuted by the Nazis from 1933 on. To put it colloquially, he was later kicked in the teeth for having risked his life for Germany.
In World War 1 (1914-18) Anne's father, Otto, had fought for Germany. After all, he was a German.