A series of swift Russian advances have challenged Ukraine’s new line of defense set up after their withdrawal from the key town of Avdiivka and are raising fears about Kyiv’s tactics and momentum on the front lines. Ukraine announced a withdrawal from Avdiivka on February 17 to a series of positions to the town’s west. Yet three tiny villages have since fallen to Russian forces, with Kyiv insisting they never intended to defend them. But the defensive line it declared it would fall back to - three villages further to the west - has since come under heavy Russian assault,...