Question

What underlying factors led to World
War I?

Answer 1

There were a multitude of underlying factors that led to World War 1. Some of these are
Assassination, Nationalism, Militarism, Imperialism, and Alliances.

Answer 2

Some damn thing in the Balkans. Was predicted to set off a war.

Everybody had treaties with everybody else. When the Archduke was assassinated, that was the some damn thing. Germany took that as a reason to invade France. Every body chose sides, honoring some treaties and conviently forgetting about others.
The whole system that was supposed to prevent war ended up solidifying most of Europe into two opposing sides. And because of European colonies, it turned into 2 large segments of the world and war.