Complex sentence: The political objective of the assassination was to break off the Austro-Hungarian's southern provinces mainly populated by Slavs from the Austro-Hungarian Empire, and it triggered a chain of events that embroiled Russia and the major European powers.
Split into simpler sentences: The political objective of the assassination was to break the Austro-Hungarian's South-Slav provinces off from the Austro-Hungarian Empire. The assassination triggered a chain of events that embroiled Russia and the major European powers.