KYIV, Ukraine — The success of Ukraine’s secret incursion into Russia is clear. Ukrainian forces pushed past two lines of Russian defenses in the southwestern region of Kursk and moved through Russian highways and villages with little resistance. Since the operation began 11 days ago, they have gone beyond Kursk to the neighboring region of Belgorod, putting other communities on edge and rattling Moscow.The ultimate strategy and goals of the invasion, though, are still murky. Western allies, including the United States and Germany, say they are watching and monitoring the situation but letting Ukraine lead the way. Even the Ukrainian...