I often wonder about the idea of US style über alles when editing endnotes.
The Chicago Manual of Style, section 17.100, states: “If the city of publication may be unknown to readers or may be confused with another city of the same name, the abbreviation of the state, province, or (sometimes) country is added.”
Who determines whether a city is likely to be unknown to readers?
I edit primarily scholarly works; for example, I would wager that many scholars know that Harmondsworth and Aldershot are in the United Kingdom (because of the number of publications coming out of those locations), but would the lay reader?
I typically leave capitals and other major cities without a country designation, as it’s reasonable to think that most people know where Madrid or Oslo or Vienna are.
But what about Valladolid? Or Bergen? Or Salzburg? Same countries as the previous cities, and yet possibly unknown to US readers. Less likely to be unknown to European readers. I know where all those cities are, so I can’t rely only on my own knowledge to decide on this.
Conversely, I would not expect a US reader to need a state for Atlanta, Seattle, or Denver, but a European reader might need it.
What the issue comes down to is whether the reader has enough information in the notes to locate the referenced publication. Now that so many references can be tracked down online with only a title, is this so crucial? I’m not sure. Even for publications with a potentially global readership.