On Aug 11, Russian soldiers breached Ukraine’s front-line defences and penetrated at least six miles behind enemy lines. Battlefield maps, kept studiously up to date by open-source intelligence groups, showed a red tendril reaching north into Ukraine’s territory through several villages in the contested eastern Donetsk region coveted by Vladimir Putin, the Russian president. The advance set off alarm bells among Kyiv’s supporters, who feared it marked the start of an offensive aimed at the stronghold city of Kostiantynivka or the logistical hub of Pokrovsk, both of which could provide Russia with a foothold in Ukraine’s fortress belt. What Russian...