"books on the wars of the 20th century could certainly do with a recognition that both WW1 and WW2 started before they were declared and continued long after they officially finished, involving people who weren't recognized as combatants by any diplomatic protocol"

Yes, but that's not really the equivalent. Have you ever read a book on World War II that dedicated a chapter to asking the question "what is a war?", before going on to provide 200 pages of potted history of other historians who've written about World War II?