Russia’s defense ministry on Saturday admitted its forces had retreated from key towns in northeastern Ukraine after a stunning advance by Ukrainian troops. The Defense Ministry said it had pulled forces out of the village of Balakliya and the strategically crucial city of Izyum in the Kharkiv region, after a decision to “regroup” and transfer them to Donetsk in the south.” The retreat represents one of the biggest setbacks for Russia since President Vladimir V. Putin launched the full-scale invasion of Ukraine in February. “In order to achieve the goals of the special military operation, a decision was made to...