Russian forces appeared poised to tighten the noose around thousands of Ukrainian troops near two strategically important cities in the fiercely contested Donbas region of eastern Ukraine on Sunday, mounting an assault on Ukrainian front lines that forced Ukraine to rush reinforcements to the area. On a day of fighting that put even territory thought to be securely in Ukrainian hands in play, Prime Minister Boris Johnson of Britain and the NATO secretary general, Jens Stoltenberg, warned that the war could grind on for years. They urged Ukraine’s Western allies to settle in for the long haul as Russia moved...