Donald Trump officially announced his candidacy for president on June 16, 2015. It was the first time he promised that, if elected, he’d see to it that the United States built a wall along its southern border. Over the next 18 months, the wall would define his candidacy. Two years into his first term, it defines his presidency still. A fight over congressional funding for the wall has triggered the longest government shutdown in the nation’s history. But the $5.7 billion price tag isn't really the core of the dispute. No, the fight became intractable because the wall has become a conceptual holy grail for the president and a stop-it-at-all-costs bugbear for Democratic leaders. It is likely to remain so for long after the shutdown ends and government work resumes. While it began in December,...