Which event had the immediate effect of causing Germany to declare war on the Allies?

Russia mobilizing its forces on Germany's eastern borders was the event that made Germany declare war on the allies.

Explanation:

The killing of Archduke Franz Ferdinand by the serbian nationalist Gavrilo princip was the trigger that started world war.The alliance system was an important factor that caused world war 1 as, any countries who were in alliance had to help the other country if a war broke out.In World War I, two major alliances faced each other.The two major alliances that fought for the power of Europe were : Allied power that includes Russia, Britain and France and the Central Powers(Germany, Austria-Hungary).