When did the 1914-18 War become "World War I"?

[FRITZ JUENGLING]

was a World War II, people wouldn't have known the Great War of 1914-1918 was
going to be just the first of at least two, and wouldn't have called it World
War I.

Well, people were smarter then than we give them credit.  A few months ago I was browsing in the military section of a used bookstore and came across a book called "The First World War"--published in the EARLY 1930's.  Unfortunately, I do not remember the exact year nor the author.
Fritz Juengling