I remember learning in school that the flashpoint for World War I was the assassination of Archduke Franz Ferdinand in Sarajevo. Like most people, I never quite understood how the first-ever World War, involving over 30 nations and leading to almost 20 million deaths, resulted from a gratuitous murder by a handful of radical students. Apparently universities should keep a very close eye on student organizations!
I later encountered the realist school of geopolitics, which argues that the Great War was a disaster waiting to happen. The actual cause of the war, according to realists, was not a random assassination, but the rise of German (and to a lesser extent Russian and American) economic and military power, which threatened the then-British-dominated world order.
Realists say this pattern is not uncommon. A number one power, alarmed at the rise of a number two challenger, allies itself with the number three power, but ultimately fails to maintain its position. The shifting power dynamics, in which the number one power no longer has the economic and military might to back up its top ranking, produces a major war, whose aftermath establishes the new international pecking order. In the case of the two World Wars, which were really one war with two major episodes, the thalassocratic British empire exhausted itself fighting Germany, allowing the US to seize the number one spot.