World War 1 started because a guy was assassinated, for one. The tension was there, but the assassination was what set everything into motion.
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").