A four-square-mile patch of Kursk Oblast in western Russia is a graveyard for Russian vehicles—and a harbinger of a looming catastrophe for the Kremlin as its yearlong offensive in Ukraine begins to falter. Kriegsforscher, a Ukrainian marine corps drone operator supporting the 20,000-strong Ukrainian force that has held a 20-by-12-mile salient in Kursk since August, tallied around 90 wrecked and abandoned Russian vehicles just in his two-by-two-mile sector on the northwest edge of the salient. That’s an entire brigade’s worth of vehicles. Ukrainian losses in the same sector have been much lighter: just 20 or so. A four-to-one loss ratio...