When was this term first used in reference to the actual WW2, rather than speculatively?

This one’s on the Big List.

Thanks, and apologies.

Big List -

The number was also added in 1914, obviously in hypothetical use indicating that the 1914-18 conflict was not the “war to end all wars.” Ernst Haeckel writing in the Indianapolis Star of 20 September 1914:

There is no doubt that the course and character of the feared “European war”...will become the first world war in the full sense of the word.

Depending on context not quoted this could be read as “the first war that is a world war” rather than as “the first in a series of world wars.” In fact, given the contrast with “European war”, I would say it is likely that this is the expected reading.

I agree with Faldage: the idea that it means “the [first world war]” rather than “the first [world war]” seems to me extremely forced.  I suggest that the 1919 date should be taken as the earliest usage.