Many historical analysis of the start of WW1 highlight the slow communication and mobilization processes as the main cause of why European military leaders thought they had to move first, just in case the other side had already started and they hadn't heard... Another way to see it is that oligarchies of the time thought it expedient to have yet another 19th century war, and didn't factor in advances in technology: machine guns, artillery, chemical warfare...
A bit like with drones and AI now: the outcome and shape of the Ukraine conflict wil inform us all of the specific bloody taste of 21st century resource wars. To anyone reading this in countries dependent on Ukrainian wheat imports (eg. North-Africa) Stock up on the pasta!
"a guy" to describe the future emperor of the Austria-Hungary is an understatement.
It's like saying that the world was shocked because in 1963 a guy was shot. Yes, technically true, but...
OP's point is that an act can seem small on the surface ("a guy got assassinated") but triggered a larger war because there are greater tensions and forces at play ("Austria-Hungary had been thinking about invading Serbia for decades").
The original "world war", the Seven Years War, started over some useless land in North America and then became global over Silesia, both of which are rather small things.
WWI started over an assassination that barely happened, and WWII over a city ( Danzig, although it's fairly certain Hitler wouldn't have stopped there, like WWI would have happened anyways).
WW1 was a war everyone wanted (partially due to the dismantling of Bismarck's complex systems of alliances). Austria-Hungary wanted Serbia (and looked for an excuse), Prussia thought war against Russia was inevitable (no idea if that's true or not) and that such a war couldn't be won by Prussia once Russia started to modernize (kind of true, as Prussia utterly defeated Russia in WW1 and Germany was utterly defeated by Russia in WW2) and thus wanted a war against Russia now. Some French wanted revanche for the war 1870. Italy had some territorial interests against Austria-Hungary. And everyone came from a world were those wars were normal (as bad as the Napoleonic wars were, they were mostly fought between armies and less severe on civilians, and decided by decisive battles with treaties after those). Everyone thought they could easily win, and fast. The trope "it's over by Christmas" comes from that period.
WW2 so, was different. That was, from the beginning, Hitler's idea. He wanted it, he took what he could without a war, and then with a war. All with the goal of defeating Russia and exterminate the Jews (and all other undesirables). Everything else was just excuses, and thinly vailed ones at that.
