The fact that Serbians assassinated the Austro-Hungarian heir is what started a war in the first place. The fact that World War I was not a small "Third Balkans War" (in three years!) is due more to the great power relations of the time. In terms of studying the causes of the war, the instructive part is not the trigger but why it expanded into war when, say, the Fashoda incident did not.

The assassination of an Austrian prince on 29th of June had very little to do with Germany invading Belgium on 3rd of August, based on war plans drawn 9 years in advance.

https://www.historyonthenet.com/world-war-one-timeline

You can use the unstable sand pile analogy to describe the state of Europe at the time. The assassination was the last grain of sand that triggered the collapse of the sand pile.

> You can use the unstable sand pile analogy to describe the state of Europe at the time.

Or, if you want to be direct a d literal, you can note that a key feature of great power relations was belief in (what is now recognized as the myth of) offense dominance—the belief that the technology of the day rendered the power that attacked first an overwhelming military advantage, such that in the face of likely impending war it was critical to attack first. It was literally an inverse of MAD.