Jack London's novel The Assassination Bureau is about the moral dilemmas facing a group of anarchists who contract out their services as assassins to clients whose motives they deem just. He did not complete the book, and it was in fact not finished and published until 1963--almost half a century later.
Naturally, it is unmentioned in discussions of the early history of the spy story, but I find myself wondering what might have been. The earlier development of the genre was overwhelmingly the work of British writers, and its initial, principal and defining tradition from Kipling, Childers and company on (to quote Julian Symons) was "conservative, supporting authority, making the assertion that agents are fighting to protect something valuable." By and large it was pop entertainment and a vehicle for nationalistic and militaristic propaganda (though Kim certainly enjoys a well-deserved status as a work of literature, as do Joseph Conrad's forays into the genre, while G.K. Chesterton's The Man Who Was Thursday likewise commands respect as "something more").
Yet in The Assassination Bureau there was an American spy novel which was unabashedly leftist and anti-militarist in its politics (enough so as to make a politically faithful film an impossibility, just as was the case with the graphic novel V for Vendetta, and for much the same reason), while also being exceptionally witty (its urbane humor surprising me as a reader who'd previously known London as the author of books like The Call of the Wild), decades before Eric Ambler came along. That London was at the time already well-known and influential made it that much more likely that, had the book been completed and published at the time, it would have been recognized.1
All that being the case, the possibility seems to me an interesting might-have-been of literary history, one that might have made the lineage of the spy story quite a different thing from what it is today.
1. The movie--which might reasonably be regarded as proto-steampunk--left the political principles of the bureau much more abstract, and played out as a Bond film in Belle Epoque period costume, complete with Diana Rigg as Bond girl, and Telly Savalas as the Bond villain (the exact roles they played in that same year's On Her Majesty's Secret Service).
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