On two separate occasions last year, railroad cars carrying tens of thousands of kilograms of smokeless powder — enough propellant to collectively make at least 80 million rounds of ammunition — rumbled across the China-Russia border at the remote town of Zabaykalsk. The powder had been shipped by Poly Technologies, a state-owned Chinese company on which the United States had previously imposed sanctions for its global sales of missile technology and providing support to Iran. Its destination was Barnaul Cartridge Plant, an ammunition factory in central Russia with a history of supplying the Russian government. These previously unreported shipments, which...