DONETSK REGION, Ukraine — Ukrainian troops are racing to claw back a key stretch of land on the eastern front after a surprise Russian advance bolstered Moscow’s position as President Donald Trump and Russian President Vladimir Putin met in Alaska. Ukraine rushed elite troops to try to seal the disastrous breach after small groups of Russian foot soldiers exploited weak points on Ukraine’s flanks and pushed more than nine miles toward the eastern city of Dobropillya in rapid gains that threaten the security of the entire contested region. If Russia cements its hold on the territory, it will damage Ukrainian...