I was in the middle of David Downing's final novel in his John Russell series set in WWII Berlin, Potsdam Station, when I read Bill Ott's excellent round-up of World War II crime fiction, A Hard-Boiled Gazetteer to World War II, in the May 1st issue of Booklist.
I made a display a couple of years ago, noticing the trend, not limited to WWII. I called it "Murder in Wartime", and I included recent crime fiction set in wars going back to the American Civil War.
There weren't many surprises for me on Ott's list. I had at least sampled most of them. Alan Furst deserves the credit for opening the vein of Nazi Noir with Night Soldiers, (1988). Philip Kerr followed soon after with March Violets, (1989).
Not mentioned by Ott is Rebecca Pawel's Sgt. Tejada series set in post-civil war Spain, beginning with Death of a Nationalist, and also Winter in Madrid by C. J. Sansom.