I've read that Austria-Hungary annexed Bosnia-Herzegovina in 1908, much to the dismay of the Serbs. Russia backed this action and was promised to have access to the Straits but were later denied. The Serbs were pressured by their Russian ally to give up their claims of Bosnia-Herzegovina (but why?). Before then, Austria had repeatedly threatened to go to war against Serbia if Serbia continued to refuse recognizing Austria's claims.
The killer who murdered Archduke Ferdinand was a member of the Black Hand, the same group that is connected with the post-revolutionary government in Serbia, so the Serbian government was complicit. From a historical point of view, it seems that Serb nationalism was the trigger to WWI.