That's rather hyperbolic. Article 4 has been invoked six times since 2003 (including in response to the Crimean invasion.) None of those cases triggered World War III.
Ukraine isn't part of NATO, and Estonia hasn't been attacked, so Article 5 can't be invoked. Therefore, this is roughly the maximally aggressive possible response from Estonia.
Of course the US Secretary of State says US commitment to Article 5 is "ironclad"; if he doesn't say that, NATO basically stops existing, and with it much of the US's influence in the world. Therefore the fact that he says this conveys almost zero information.
Because if Putin was to attack the Baltics tomorrow, they would cease to exist before any NATO ally would manage to get their warships past the Kaliningrad Oblast in the Baltic Sea.
Austria-Hungary had been thinking about invading Serbia for decades.
I dislike this historical view where the little guy is blamed.
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").
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).
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.