Ukraine’s most sophisticated attack drone is about as stealthy as a crop duster: slow, low-flying and completely defenseless. So when the Russian invasion began, many experts expected the few drones that the Ukrainian forces managed to get off the ground would be shot down in hours.

But more than two weeks into the conflict, Ukraine’s drones — Turkish-made Bayraktar TB2 models that buzz along at about half the speed of a Cessna - are not only still flying; they also shoot guided missiles at Russian missile launchers, tanks and supply trains, according to Pentagon officials.

The drones have become a sort of lumbering canary in the war’s coal mine, a sign of the astonishing resiliency of the Ukrainian defense forces and the larger problems that the Russians have encountered.