A special report explains, Ukraine’s killing fields hold three big lessons. The first is that the battlefield is becoming transparent. Forget binoculars or maps; think of all-seeing sensors on satellites and fleets of drones. Cheap and ubiquitous, they yield data for processing by ever-improving algorithms that can pick out needles from haystacks: the mobile signal of a Russian general, say, or the outline of a camouflaged tank. This information can then be relayed by satellites to the lowliest soldier at the front, or used to aim artillery and rockets with unprecedented precision and range.