Ukrainian military commanders said on Wednesday that their troops had broken through Russian positions on the southern flank of the embattled eastern city of Bakhmut, forcing Russian units back from their positions at an important bridgehead of a canal. Ukrainian officials and the head of Russia’s Wagner militia group said that Russian troops had lost an area of roughly three square miles southwest of the city. If confirmed, it would be the first significant gain for Ukraine in the fight for Bakhmut since pushing Russian forces off a key access road two months ago, although it was far from clear...