Historical event that acquired its name almost two decades after it ended, in brief

WWI

["This global conflict was initially called the Great War before its numerical label."]

This clue refers to World War I, which wasn't called that until after World War II began in 1939. The "in brief" part hints at the abbreviation "WWI," and the name stuck retroactively. It’s a straightforward reference to how historical events are often labeled long after they happen.