Do We Have This One?
where in fiction during a great war there's a certain strategic loss that is very significant to the point of being demoralizing, shocking, and Iconic in universe? For instance Star Wars
have the destruction of the first death star, Halo universe has the fall of reach. Sometimes it could be a turning point in the war, but not always. A faction can suffer these defeats and not necessarily be defeated. But it's still a crippling loss that'll at least slow them down significantly. Sometimes these defeats are due to surprise attacks. Sometimes it's described as a Noodle Incident
in some stories.