Austro-Hungarian troops begin to cross the Drina from Bosnia into Serbia. The reckoning with Serbia has begun. The Austro-Hungarians are commanded by Oskar Potiorek. As military governor of Bosnia-Herzegovina, he had been in the car with Franz Ferdinand when Gavrilo Princip fired his fatal shot. Now Potiorek hopes to have his revenge on the Serbs.
The invaders soon encounter opposition from irregular Serb forces. They respond with brutal reprisals.