Most people think of World War I as a tragedy, because it didn't need to happen, and it didn't accomplish much except for creating social and economic conditions that made World War II possible. So when we're talking about causes of the war, inevitably we're also assigning blame. The immediate cause was, of course, the assassination in Sarajevo of the Austrian Archduke Franz Ferdinand on June 28, 1914 by a Bosnian-Serb nationalist named Gavrilo Princip. Quick aside, it's worth noting that the first big war of the 20th century began with an act of terrorism. So Franz Ferdinand wasn't particularly well-liked by his uncle, the Emperor Franz Joseph. Now that is a mustache. But even so, the assassination led Austria to issue an ultimatum to Serbia, whereupon Serbia accepted some, but not all, of Austria's demands, leading Austria to declare war against Serbia.