Marching quickly under a cloudy sky on Sunday, Russian troops infiltrated Avdiivka. Four months after a pair of Russian field armies, together with 40,000 troops and thousands of vehicles, first attacked toward Avdiivka—a Ukrainian stronghold just northwest of Russian-occupied Donetsk in eastern Ukraine—the long, bloody campaign may be culminating. “The situation in the city has become critical,” Ukrainian journalist Andriy Tsaplienko wrote. If the campaign does culminate, and the Russians prevail, it will be clear who largely is to blame: Russia-aligned Republicans in the U.S. Congress, who last fall began blockading U.S. aid to Ukraine and, as a consequence, starved...