Moscow abandoned its main bastion in northeastern Ukraine on Saturday, in a sudden collapse of one of the war's principal front lines after surging Ukrainian forces threatened to encircle the area in a shock advance. The swift fall of Izium in Kharkiv province was Moscow's worst defeat since its troops were forced back from the capital Kyiv in March, and could prove a decisive turning point in the six-month-old war, with thousands of Russian soldiers abandoning ammunition stockpiles and equipment as they fled. The head of Russia's administration in areas in Kharkiv it controls told all residents to evacuate the...