Russians first brought fiber-optic drones to the war this past spring, and since then, Ukraine has been racing to develop and produce them at scale. The weapons became more prominent in the fall, especially in Russia's Kursk region, where Ukrainian forces launched a shock invasion in early August.

Russian drones are a constant threat in besieged Pokrovsk. WSJ’s Ian Lovett reports from the city where some residents are determined to remain, despite Moscow's forces on their doorstep.