Answer:
The spark was the assassination of the Austro-Hungarian Arch Duke, Franz Ferdinand, by a serbian nationalist, in the city of Saravejo.
Explanation:
This assassination created a conflict betweent the powerful Austro-Hungarian Empire, and the weak and much smaller Serbia, something that prompted Russian intervention on behalf of Serbia.
Germany soon followed, and declared war on Russia because it was an ally of Austria-Hungary.
Finally, France and Britain declared war on Germany because they were allies of Russia.
