After deciding that the heroine of my newest historical mystery series—Verity Kent—had worked in the foreign division of the British Secret Service during the Great War, it wasn’t long before I determined I also wanted her to have served in some capacity abroad, perhaps even behind enemy lines. Uncovering exactly what role I wanted her to play as a field agent was the trickier part, particularly as I wanted it to be realistic. But it was as I was perusing the history section of...