Why did European countries join the world war 1?

European countries joined World War I due to a complex web of alliances, militarism, nationalism, and imperial ambitions. The assassination of Archduke Franz Ferdinand of Austria-Hungary triggered a chain reaction, as countries were bound by treaties to support their allies. Nationalistic fervor fueled the desire for territorial expansion and dominance, while militarism led to an arms race that heightened tensions. Ultimately, these factors combined to pull multiple nations into a conflict that escalated rapidly.