Unthinkable Wings

I wandered off and had a look at some of the comms in question, and actually got boggled off my mounts by some sanctimonious poster talking about the Eval of Austria "starting the first World War just because a single arch-duke was shot". It left me wondering; is there a defined minimum number of arch-dukes whose assassinations justify a global conflict? It also occurred to me, depressingly, that the poster clearly had no idea whatsoever who Franz Ferdinand was, or, indeed, that Austria was something rather more like the US in size and influence at the time, rather than "quaint little landlocked country in Central Europe; principal exports yodels and Von Trapps."