If I were to be completely honest with myself, I'd admit that because Necessary Evil is the final volume of a trilogy, the story behind it is, technically, the story of its preceding volume, The Coldest War. And the story behind that is the story behind the first book of the trilogy, Bitter Seeds. . . Meaning the story behind Necessary Evil is the story behind the entire Milkweed Triptych.
In Joseph D'Lacey's description of the story behind Black Feathers, he describes how a collision of several unrelated ideas sometimes gives rise to something bigger than the sum of the parts. I formulate novels in a similar fashion. (Or, in this case, a trilogy. But more on that in a moment.)
The first seed for what eventually became the Milkweed books was planted in my imagination when I read about a truly bizarre piece of World War II trivia called Project Habakkuk. It was born during the darkest days of the Battle of the Atlantic when German wolf packs were wreaking havoc on British shipping. The Admiralty, perhaps somewhat desperately, contemplated building ships from ice. Alas, the project never made it past the prototype stage. But in my imagination I saw vast bergships plying the North Atlantic. And I wondered just how the Axis might have responded if Habakkuk had been successful.
The answer came to me in a flash: "Obviously, Ian, the Third Reich would have sent a pyrokinetic spy to sabotage the icy shipyards." And in that moment I saw the spy, and his world, and the program that created him. I played with this fictional setting, and populated it with jaded British magicians and superpowered German agents. Including a woman named Gretel.
Gretel grew directly from a second seed, which was planted in my imagination by -- of all things -- the Tom Cruise movie Minority Report. It features a scene where the hero is on the run but just happens to be accompanied by a woman who can see the future. Her prescient advice keeps them 30 seconds ahead of their pursuers, enabling their escape. I thought that was magnificent! But later, I thought, "Hmmm. 30 seconds isn't bad. But a really good precog would be 30 years ahead of everybody else."
As I thought about these pieces, I slowly came to the conclusion that this world was too big for a short story, or even a succession of stories.
Meanwhile, the third and fourth seeds of what grew into the Milkweed books were planted by my local writing group. Our group has a "pay to play" rule, meaning that in order to participate in the monthly critique discussions one has to submit their own work for critique as well. (That way nobody gets to take free pot shots at the others!) I'd been in the group about a year, and had spent that time feverishly devising a new short story every month. Eventually I decided that it would be a better use of my time -- that I'd learn more -- if I could spend a year on a single project. A trunk novel was just the thing: something written entirely for practice, never intended for publication, so that I could focus on my craft without worrying about a new plot each month.
So I rummaged the ratty, weed-choked, untended garden of my imagination for a novel-sized idea. And, somewhat reluctantly, I brought my idea for a blood-magic-and-superpowers take on World War II to the writing group. "I know this is a stupid idea, guys, but do you think it would make a decent practice novel?"
They greeted the idea with enthusiasm (to my everlasting shock). But the concept I presented to the group was a single novel set during the 1960s because the concept most interesting me featured retired spies living decades after the events of a fantastical alternate-historical war. To which they said, "Ian, you want to write a novel about the Cold War. But it's predicated on an alternate World War II with superpowers and black magic and alternate history. You cannot fill that in as backstory! Are you nuts?"
Thus they convinced me that in order to write the book I really wanted to write -- which eventually became The Coldest War -- I first had to write an entire novel about the war itself. That became Bitter Seeds. And, of course, as the triptych structure of the story asserted itself, we knew it would require a concluding volume to see it properly finished. Which, of course, is how Necessary Evil came to be.