In early September, Ukrainian forces began deploying hovering drones equipped with tanks full of thermite, a metal incendiary that sprays like a liquid and burns as hot as 5,000 degrees Fahrenheit. Russian forces cobbled together their own versions of the Ukrainian “dragon drone” a few weeks later.

It looked like an ordinary, modest house on the outskirts of Sloviansk, a small city just behind the front line in eastern Ukraine. But the parlor’s heavy furniture had been