Serbia has already repelled an Austro-Hungarian invasion. Its forces have also crossed the Sava river and occupied the Austro-Hungarian town of Zemun. Many ethnic Serbs live in Zemun. Now they flee into Serbia proper, fearing reprisals should Austria-Hungary retake the town.
The Serbian army also crosses the border into Bosnia-Herzegovina. Here the Serbians are not alone, for the invasion force is aided by soldiers from Montenegro. The small mountain kingdom has entered the war on the side of their Serbian allies. Montenegro is a poor country, but its men have a reputation as fighters of great ferocity.