>>You could argue that the entirety of Europe declaring war on itself over the death of one royal (and not even a reigning monarch; an heir-apparent) is such an example

I mean that was just an excuse, in hindsight it's completely obvious that Europe was gearing up for war for years prior to the event. Just like now it seems completely possible that we might end up in a war or even civil war in some countries over a (seemingly) minor event - it's just going to be a spark that sets off the powder keg.