What critical role did the balkans play in the start of world war 1?

The Balkans Campaign  of World War I was fought between the Central Powers, represented by Austria-Hungary, Bulgaria, Germany and the Ottoman Empire on one side and the Allies, represented by France, Montenegro, Russia, Serbia, and the United Kingdom (and later Romania and Greece, who sided with the Allied Powers) on the other side. The Balkans were a cluster of nations in eastern Europe, between the Austro-Hungarian and Ottoman empires. 2. Their location made the Balkans strategically important, so European powers were focused on events there. ... The Balkans were disrupted by two wars in 1912-13, as well as rising Serbian nationalist groups.

The assassination of Archduke Franz Ferdinand in the Balkan city of Sarajevo provided the Austro-Hungarian government with a pretext for crushing Serbian nationalism, something it had long desired,
 while nobody seemed to much care about the assassination itself, Austria-Hungary had been looking for an excuse to wage a “preventative war” against Serbia as a state in order to weaken or destroy them so as to take back territory in the Balkans, which had been taken during the Balkan Wars.  They had not taken it back up to this point because they lacked Germany’s support;

Except common history