BAKHMUT, Ukraine — Surrounded on three sides, this eastern Ukrainian city has held out for months in bloody defiance against invading forces, Russian shells pounding day and night against streets coated with snow and rubble. But, as the Kremlin’s troops advance here and threaten a major offensive across the war’s front lines, gunfire could be heard on a recent visit while the few thousand remaining locals sheltered in basements, stubbornly refusing to leave what remains of their homes. "Bakhmut holds" is a patriotic battle-cry heard across the country, cementing the city as a symbol of national resistance. And it still...