Answer:
Murder of franz Ferdinand and his wife.
Explanation:
For a while then, Serbia and Austria Hungary had a bitter relation. Austria Hungary had, at that time, annexed bosnia and herzegovina.
When franz, the crown prince of Austria Hungary and his wife were killed in Bosnia, Serbia and the black hand society were suspected and blamed.
Thus, the king of Austria Hungary sent a list of unfulfillable demands to Serbia and when serbia denied, Austria Hungary declared war on serbia which can be marked as the start of world war.
this is how the murder of the prince was a breaking point of the war
