The Balkans was a region of Europe that had been a hotbed of tension for decades. The region was home to a number of different ethnic groups, each with its own language, culture, and religion. These groups had often been in conflict with each other, and the region had been the site of several wars in the past.
In the years leading up to World War I, the Balkans were in a state of flux. The Ottoman Empire, which had controlled the region for centuries, was in decline, and several new nation-states had emerged in the region. These new states were often at odds with each other, and the region was a tinderbox of tensions.
The spark that ignited World War I was the assassination of Archduke Franz Ferdinand of Austria, the heir to the Austro-Hungarian throne. The assassination was carried out by a Serbian nationalist in Sarajevo, the capital of Bosnia-Herzegovina (which is now part of Bosnia and Herzegovina).
Immediately after the assassination, Russia declared war on Germany and the conflict escalated into World War I.