http://www.history.com/topics/the-de...f--world-war-i
On June 28, 1914, a young Serbian nationalist named Gavrilo Princip killed Archduke Franz Ferdinand, heir to the throne of the Austro-Hungarian Empire, in Sarajevo, Bosnia. Taking place against a backdrop of escalating tensions in the Balkans, the assassination set off a chain of events that would lead to the start of World War I barely one month later. To many people, the Great War—as it was known at the time—seemed to come out of the blue, as the European continent was enjoying a long stretch of unparalleled peace and prosperity. In fact, the seeds of the devastating conflict had been planted long before Princip fired those fatal bullets.
When the world situation gets to the point where all it needs is a match , things go downhill fast. It only took a month after the Archduke was killed for the world to be at war.