Europe's Last Summer: Who Started the Great War in 1914 by David Fromkin

(He's the one who wrote The Peace to End all Peace about the Middle East, the end of the Ottoman Empire and the emergence of the Middle East as we know it.)
He says the best way to understand this question--and Barbara Tuchman in The Guns of August didn't have enough data to do so--is to realize there were two wars. Austria/Hungry wanted to get rid of Serbia who was a threat and so blamed the death of Arch Duke Franz Ferdinand on Serbia (the assassination occurred in Sarajevo) and declared war. That was one war. Germany wanted to go to war to establish its position as a major European power and especially von Moltke, the head of the army, had determined that sooner was better before Russia, France and England became too strong. So Germany encouraged its ally Austria in its war. Austria's war with Serbia met with defeat but soon von Moltke manoevered (because Kaiser Wilhelm was actually against war) Germany into war with Russia and its ally France. Britain joined when the Germans attacked Belgium but also because it was concerned the Germany would overpower its ally France. The second war, started by Germany, overwhelmed the first one. (If my explanation seems confusing, read the book. It's very complex and I'm not sure I have it straight, but the answer is that Germany started WWI.