Its been Teuton vs Slavs for a long time. The estimate is made that Europe contains in all about 140,000,000 Slavs, this being the most numerous race on the continent, the Teutons ranking second. The Slavs are mostly in Eastern Europe and they feel connected by blood (well, only Bulgaria is the one slavic country that is a traitor), helping each other. For ex. it doesnt matter what kind of problems the polish provinces of that time had with Russia, they still share the hatred towards the Germans and will stand together. Russia has always been our big-brother. Now, the Teutons as you know are spread to the west and north. But they dont have that PAN-teutonisam as the slavic people had. They didnt feel close by race, didnt have community and fraternity, instead there was racial divergence. At that time the Pan - slavic movement is very strong with determination for a great slavic nation under russian rule. So, the Teuton is going against the Slavs to defend themselves from the slavism. So, with all this boiling, Serbias independance threatened by Austro-Hungaria is the perfect reason for Russia to enter war