First use of the expression "Spandau Ballet"

The German Wikipedia article gives the following reference for the WWI military jargon:

Warlord Games (2016). Bolt Action: Armies of Germany: 2nd Edition. Bloomsbury. p. 33. ISBN 147281780X.

I presume this work, if you can find it in a library, would help you further.

This alternative aetiology for the band name assumes that Elms simply invented the graffiti story, since a Berlin bathroom wall at the time would certainly be scrawled with something about the prison.

A query of the British Newspaper Archives, the Library of Congress newspaper archive, Elephind (Australian and US newspapers), as well as the Hathi Trust archives and Archive.org turned up no references other than to the band.

This doesn’t mean that the jargon term didn’t exist, merely that it didn’t escape into the daily press during the war or later, at least among those newspapers and other works available online. Given the gruesome and dehumanizing nature of the image, perhaps that isn’t surprising.

This, of course, raises the question of where and how any member of the band might have heard the expression in a military context and then decided that, even though describing wholesale slaughter during trench warfare, it sounded so cool they'd use it anyway.

I couldn't find any textual references older than than the band which is a bit strange if Wikipedia's info is correct (or I'm just looking in the wrong place). The Wiki entry on the district of Spandau in Berlin says the phrase dates from WWI.

Other landmarks include [...] Spandau arsenal, Germany's arms development center until 1919, now a museum. That arsenal's Spandau machine gun inspired the slang Spandau Ballet to describe dying soldiers on barbed wire during the First World War, and later was applied to the appearance of Nazi war criminals hanged at Spandau Prison.

It wasn't actually clear to me if the slang was originally German or English slang. The German Wiki entry says it was the allied soliers who coined it. I'll add the comment for the sake of completion.

Alliierte Soldaten nannten die Bewegungen von Leichen, die im Stacheldraht von Schützengrabensystemen hängend von deutschen Spandau-MGs getroffen wurden, "Spandau-Ballett".

I doubt that "Spandau ballet" appeared in English before the New Wave-ish band The Gentry (formerly the Makers, formerly The Cut) changed their name to Spandau Ballet in 1979 or 1980.

If the term "Spandau ballet" did appear on a bathroom wall in Berlin in 1980, it might have referred to Spandau in the sense of the Spandau MG 08 heavy machine gun, which the German military produced beginning in 1908—and thence to the behavior of human beings shot by it—or it might have referred to Spandau Prison near Belin, which opened in 1876 as a military detention facility, became a prison for civilian and military inmates in 1919, and became a prison reserved for Nazi war criminals after the Nuremburg trials in 1947.

According to the Wikipedia article on Spandau Prison,

The British band Spandau Ballet got their name after a friend of the band, journalist and DJ Robert Elms, saw the name 'Spandau Ballet' scrawled on the wall of a nightclub lavatory during a visit to Berlin. This gallows humour graffiti refers to standard drop method hangings at Spandau Prison when the condemned would twitch and jump at the end of a rope.

The authority cited for this paragraph is True: The Autobiography of Martin Kemp, p. 44. Martin Kemp is one of the central members of the band Spandau Ballet.

The reference to hangings at Spandau Prison evidently doesn't involve anything that occurred during the 1947–1987 period when a total of seven Nazi war criminals were held at Spandau Prison, because none of those seven men were executed.

I have no idea whether "Spandau ballet" has had a life in German from the days of World War I or even from the earliest days of Spandau Prison in 1876. But in English databases, I couldn't find any record of it from before 1980 (at which point the first references are to the band Spandau Ballet).

I checked a couple of generally reliable dictionaries that emphasize war slang— Paul Dickson, War Slang: American Fighting Words from the Civil War to the Gulf War (1994) and Eric Partridge, A Dictionary of Slang and Unconventional English, fifth edition (1961)—and they have nothing for "Spandau ballet" or "ballet" of any other kind. Partridge, who has a particular interest in British military slang, does have an entry for spandau, but it involves a very specific, nonlethal meaning:

spandau or Spandau. Generic for the latrines at Ruhleben internment camp, 1914–18. Ex the 'mushroom' munition-town of Spandau.

If British troops had used the term "Spandau ballet" in World War I as common slang, I am confident that Partridge would have included it in his dictionary. That he did not—and that Dickson did not on the American side—suggests that there was no popular slang term "Spandau ballet" in English during this period.

The history of "Spandau ballet" in German (assuming that it has one) is, I think, off topic at English Language & Usage. The history of the term in English appears to begin in 1980 and to plunge almost immediately into competing pseudo-folk etymologies.