Complex sentence: This became known to the Austrians at the beginning of World War I, when their army temporarily took Loznica in western Serbia, where they found documents of National Defence containing records of all Bosnian attendees of the drills.
Split into simpler sentences: This became known to the Austrians at the beginning of World War I, when their army temporarily took Loznica in western Serbia. There they found documents of National Defence containing records of all Bosnian attendees of the drills.