We have a question that has been marked as a duplicate. The question is:
The suggested duplicate is here:
The problem is that the question is not actually a duplicate, and the suggested answer on the second question doesn't answer the first.
The suggested answer quotes an answer from English:SEW
OED says that the Manchester Guardian coined "World War No. 2" on 18 February 1919, "with reference to an imagined future war arising out of the social upheaval consequent upon the First World War (1914-18)."
That mention of The First World War is a commentary on the answer by the Oxford English Dictionary. In fact, the article in Manchester Guardian doesn't mention "World War 1" or the "First World War" at all (the newspaper is hosted on Newspapers.com. It requires a subscription to view, but they offer 7 days free access if you register).
In fact, The first public use of the phrase "First World War" seems to have been in the title of memoirs published in 1920, and the first public use of the phrase "World War 1" appears to have been by Time Magazine in June 1939. However, this still doesn't quite answer the question. In 1920, and even in 1939, the term "The Great War" was still far more common in general use.
Interestingly, various people have done analyses to discover when the terms "First World War" and "World War 1" became more popular than "The Great War" in general use. I wasn't able to find one that would stand up to rigorous academic scrutiny (sources of potential bias in the data are obvious in every example I found), but this article does contain a nice interactive graph.
The suggestion is that use of "First World War" and "World War 1" overtook the phrase "The Great War" in general use in about 1941.
Interestingly, the question is not as trivial as it first appeared.
Which is all fine and, if the question hadn't been closed as a duplicate, I might well have posted it as an answer. Which brings me to my question:
Why was the question closed as a duplicate in the first place?
EDIT: I'm happy to report that the question has now been re-opened.