DONETSK REGION, Ukraine — The Ukrainian sergeant slid the captured Russian rocket launcher into the center of a small room. He was pleased. The weapon was practically brand-new. It had been built in 2020, and its thermobaric warhead was deadly against troops and armored vehicles. But the sergeant, nicknamed Zmei, had no plans to fire it at advancing Russian soldiers or at a tank trying to burst through his unit’s front line in eastern Ukraine. Instead, he was going to use it as a bargaining c...