From Cambridge Dict:
He wrote a book on Barcelona during the Spanish Civil War.
It seems "during the Spanish Civil War" can mean both the action of writing the book happened during the Spanish Civil War and the subject of the book is the city, Barcelona, during the Spanish Civil War.
How to get the true meaning of it?
This is a common ambiguity in English. Prepositional can modify different parts of a sentence. Compare the joke "This morning I shot an elephant in my pyjamas.... I don't know how it got in my pyjamas!"
Your example can be interpreted in two ways. Grammar can't decide which one is correct. It could be adjectival, describing Barcelona. It could be adverbial describing when it was written.
Both bracketing are valid. You cannot deduce which is correct by considering only the grammar.
He wrote [a book [on [Barcelona during the Spanish Civil War]]].
He wrote [a book [on Barcelona]] [during the Spanish Civil War].
And consider that
The Spanish islands were not badly affected. Baedekers wrote a travel guide on Madeira during the Spanish Civil War that was published in 1939.
But if you know that Barcelona was a major battlefield of the Spanish civil war, it is very unlikely that it means he wrote the book during the war. It is far more likely that is about the war, and so the phrase is adjectival.
You get the true meaning of the sentence from your knowledge about history, and applying common sense, not your knowledge of grammar. This is called "pragmatics".
You use pragmatics all the time in your native language, without even thinking about it!