Archduke Franz Ferdinand was assassinated in June of 1914. After much hand-wringing and indecision on the proper response, Austria-Hungary declared war on Serbia in August of 1914. Europe was entangled in a series of complicated alliances that involved both Serbia and Austria. Serbia was allied with Russia, which was allied with France. While Britain was in agreement with, but not yet in a formal alliance with France, Austria and its ally Germany formed the Central Powers, and by the end of August 1914 all these nations were at war.
This is happened in :
a. World war I
b. World war II
c. World war III