It was a warm spring evening, and I had just crossed the tarmac at Kigali International Airport and cleared customs on my way out of Rwanda. The air smelled less like jet fuel and more like air—fresh, untainted, clean. This may have been the result of President Paul Kagame’s environmental advocacy work, or perhaps something else. It had been raining on and off throughout the day, sometimes a light drizzle, at other points a sustained downpour, and I thought, not for the first time, how disconcer...