On a clear day in the spring of 1862, two orbs rose quietly into the skies above southeastern Virginia. For the soldiers in the Confederate encampment below, it must have been a wondrous, if demoralizing, sight. These strange forms were Union hot-air balloons, each with two observers in the basket, spies in the firmament. If the observers had cameras--and word was that they did--they would be able to produce perfect records of what they saw, instant maps of the Confederate positions that would soon, no doubt, be conveyed to the upper reaches of the Union Army''s chain of command. "We watched with anxious eyes their beautiful observations as they floated high up in the air," Confederate general James Longstreet later wrote of the encounter. To Longstreet and his troops, it was clear they were witnessing a turning point in the history of war. It was also clear that there was nothing they could do about it. The balloons, Longstreet lamented, were "well out of range of our guns.

" One hundred and fifty-five years later, on a searingly hot afternoon in June 2017, Steve Suddarth, a former US Air Force colonel, is radioing air traffic control for clearance to take off from Albuquerque International Sunport, the city''s main airport. The flight, he tells the tower, is a "photo mission." Riding shotgun in Suddarth''s white single-prop Cessna, I am not sure I would have used such a mild term to describe our plan. The plane is equipped with a military-grade surveillance camera, and we intend to watch wide tracts of the city in a way many of its residents probably never imagined was even possible. Air traffic control grants us free rein within a broad swath of airspace 12,000 feet directly above the city center. As we climb out of the airport through the choppy desert air, the landscape falls quickly from our wings. Soon enough, the entire town lies rolled out below us, shimmering in the afternoon sun. Mounted on the windshield is a tablet displaying a satellite map of the city.

Using a wireless keyboard he keeps stowed beside his seat, Suddarth clicks on a large white building in the center of town and a new image appears on the screen. It looks like a satellite picture, superimposed perfectly over the first, showing all the same buildings and roads. But when Suddarth zooms in, the image becomes like a sample of pond water seen through a microscope--full of life. There are cars, trucks, and buses on the streets, stopping and starting at traffic lights, turning at junctions, parking along sidewalks. Everything is moving. This is live video delivered from the camera. The frame covers two-dozen city blocks. Suddarth explains that the camera is linked to the Cessna''s autopilot, which is programmed to steer the airplane in such a way that the target area never falls out of the frame.

To demonstrate, Suddarth takes his hands off the controls, and without pause the airplane pitches left. Once the airplane finds itself in range, he says, the camera will put us into a wide circular orbit. Our particular target--a mall, Suddarth tells me cheerfully--remains dead center on the screen. After completing the loop, we swing the camera over to the University of New Mexico. On the athletics field, tiny shadows dart about on the screen: students exercising. From the university, we turn to Sandia Heights, an affluent neighborhood on the northeastern edge of the city. Using the tablet, I zoom in on a four-lane road cutting across the area. A bright blue car is turning onto a tree-lined street.

I follow it. Suddarth is talking about the system''s technical details, but I am absorbed in the story unfolding before me. The car is driving slowly, and it seems to be meandering. Maybe they''re lost, I think, or perhaps they''re on a joyride. Or maybe they''re up to something. And then, sitting in this odd little plane steered by a robotic camera, I start to feel uncomfortable. The people in the car don''t know they are being watched. Even if they did, like Longstreet and his soldiers there is not much they could do about it.

More to the point, I am troubled because, whatever their story, it is none of my business. The story of how we got here, though, is my business, and theirs too. Even before 1862--before, even, the advent of manned flight--it was obvious that an eye in the sky would be a great military asset. As seen from above, your adversary is an open book. You can determine where they''re hiding, track them wherever they go, even anticipate their next move. By the First World War, every major military power had eyes in the sky, with dramatic consequences for those on the ground.