I understand that the murder of Franz Ferdinand (and his wife) was not the sole reason for Austria-Hungary's decision to invade Serbia, but it was certainly a substantial factor.
Since that time, I am curious to know if any other nations have cited the murder of one or two of its citizens as a reason for war, as opposed to a military or mass casualty attack.
Nations have gone to war over lesser injuries to their citizens than murder.
In 1739, Britain declared war on Spain. This was "The War of Jenkins' Ear". The Spanish coast guard had boarded a British merchant ship commanded by Captain Robert Jenkins, to see if the ship was breaking the terms of the "Asiento" (a treaty which allowed British merchants to trade specified amounts with the Spanish colonies).
In the course of trying to get Captain Jenkins to admit to breaching the Asiento, the Spanish coast guard cut off the Captain's ear. Jenkins testified in Parliament about his treatment by the Spanish, including displaying the severed ear.
Britain declared war immediately!
This was the war which gave birth to the song, "Rule Britannia!".
The War of Jenkins' Ear became subsumed into the larger War of the Austrian Succession in 1742, which did not end until 1748.
So, Take that Spain! If you cut the ear off of one of our nationals, we will fight you for nine years!