What or who was the causal factors of World War 1?

In very simple terms:

The Archduke of Austria Franz Ferdinand was assassinated by a Serbian which caused Austria-Hungary to make a list of demands to end Serbia's influence in Bosnia. The Serbians rejected the demands and Austria-Hungary declared war on Serbia.

Unwilling to lose their own influence in the Balkans Russia mobilized for war with Austria-Hungary. Germany, Austria-Hungary's ally declared war on Russia.

France who were going through government instability saw the situation as a chance to use war to enact reforms and retake the Alsace-Lorraine province lost to Germany in the Franco-Prussian war and so France mobilized against Germany.

Great Britain Entered the war when Germany refused to guarantee Belgium's neutrality and sovereignty.