A question from Yahoo! Answers:

Why did the assassination of Franz Ferdinand lead to WW1?
or why did they assassinate him?

It didn’t. What really led to WWI was the gold standard. I’ve written about this repeatedly. See this for example…

Assassination of Franz Ferdinand was just a pretext. If it didn’t happen, another pretext would be found in a short order… Recall that ultimately, WWI was not about Austria or Serbia; it was about Germany and its ability to feed 40 million people while having population of 63 million. This was the real problem German government faced, and being comprised of military aristocracy, it knew only one way of solving it, through war. German exports had to be forced on France and Britain (and gold thus obtained used to buy food in Argentina and Australia), while German agriculture had to be expanded into the Ukraine to lessen the need for food imports. The alternatives (such as industrialization of agriculture) were far less attractive, because they would lead to the rising prominence of German commercial class and the eventual loss of political power by the military aristocracy (German aristocrats needed only to look at Britain and France to see what history has in store for them).

As to why Franz Ferdinand was assassinated, that’s a convoluted story…

…According to the provisions of the 1878 Treaty of Berlin, Austria-Hungary occupied and administed the former Ottoman provinces of Bosnia and Herzegovina. Serbia, meanwhile, was recognized as a sovereign state. The problem was that this arrangement left plenty of Serbs outside Serbia. For a while, nobody did anything about it, but in 1903, King Alexander and Queen Draga of Serbia were killed during a palace coup. The Obrenović dynasty to which King Alexander belonged ended; the conspirators installed Peter Karađorđević (who lived in exile at the time) as the new king crowned Peter I (a side note: the blood feud between the Karađorđević and Obrenović families goes back to 1817, when Đorđe Petrović, aka Karađorđe, the founder of the Karađorđević dynasty, was killed by Miloš Obrenović).

Peter reversed pretty much every policy that Alexander had in place. His ultimate goal was to expand Serbia to its 14th century borders. In that, he got a lot of moral support from faraway Russia, whose government would gladly underwrite anything that could spite the Austrians as long as it wasn’t too expensive. France, where Peter lived most of his life (and even served in the French army during the Franco-Prussian war of 1871) also supported him, but less openly.

As a result of two Balkan wars (1912-1913), Serbia reclaimed Macedonia and Kosovo from the Ottoman Empire and Bulgaria. Bosnia and Herzegovina were next in line. Population there was not particularly happy with the Austrian rule, which led to a series of peasant riots, the last one happening in 1910. But Franz Ferdinand, the heir to the Austrian throne, came up with a plan than could eventually make union with Serbia unattractive to Bosnia and Herzegovina. He thought that all Slavic lands under Austrian rule could be reorganized into the third kingdom within the empire (Austria and Hungary being the first two). Had that come to pass, Bosnian Serbs and Muslims would no longer be second-class citizens, which could render Serbian unification rhetoric significantly less attractive. As a result, Serbian unionists perceived Franz Ferdinand as a threat to their agenda; eventually, colonel Dragutin Dimitrijević (aka Apis), head of Serbian military intelligence, tentatively tagged him for elimination.  A small group of (mostly Bosnian) operatives was formed to assassinate Franz Ferdinand or, failing that, general Oskar Potiorek, the Austrian governor of Bosnia and Herzegovina…