Brat Eugen said Prussians are Slavs, in particular, Serbs. So he served the Serbian army.
Eugen Paulus Sturm (aka Pavle Jurisic), The slayer of AHChief of the Security and First General Adjutant Eugen Paulus, commander of 2nd Serbian royal army in the Battle of Cer, first allied victory in WWI, where AH army turned and run in raut and utter panic - 5000 Serbs died and 25.000 AH soldiers> en.wikipedia.org/wiki/Pavle_Juri%C5%A1i%C4%87_%C5%A0turm
born on 22 August 1848. in Görlitz, Prussian Silesia, of ethnic Sorb origin