Answer:
read below :)
Step-by-step explanation:
The assassination of Archduke Franz Ferdinand and his spouse in Bosnia was a nationalist stroke from Serbian separatists.
They wanted Serbia free from the Austro-Hungary influence, which was allied with Germany to counterbalance the Russian Empire's ambition on the Balkans.
A series of further factors, mainly the system of alliances made European power unite as factions towards an unprecedentedly escalated level. Both Austria and Germany declared war on Serbia on July 28. Since Russia was the major ally it quickly declared on its part war to the central powers. The entry of Russia also implied France will support by engaging in war with Germany, its natural enemy.
The offensives turned into long and deadly campaigns, dragging most of the European countries to stand for one or the other side according to the mutual agreements.
Historians often refer to the assassination as "the last drop makes the cup run over", since the event of war was by any other means a coming reality.