Russian forces have made rapid gains in the eastern Donetsk region over the past week or so, capturing a few villages and closing in on the city of Pokrovsk, one of the main Ukrainian defensive strongholds in the area. Russian forces are now only a dozen miles from Pokrovsk after Moscow’s troops pushed along a railway line and advanced about three miles toward the city, according to open-source maps of the battlefield based on combat footage and satellite imagery. The Russian progress contrasts sharply with the slow but steady gains that Moscow had made so far this year in the...