We had no preference in Sunday’s Colombian presidential election runoff and frankly we didn’t envy Colombians the decision they had to make. After a first-round election on May 29, the voters’ choices had been narrowed to two former mayors, whose competence was iffy and whose ideologies were extreme when they were not vague: populist 77-year-old construction magnate Rodolfo Hernández, a Trump-like independent, and Gustavo Petro, a leftist senator and former militant in a guerrilla group. Mr. Petro emerged as the winner with just over 50 percent of the vote. Thus does Colombia, a pillar of stability in recent Latin American history, join a regional trend that has seen traditional center-left and center-right parties nearly collapse — and leftists capture presidencies in Peru, Chile and Mexico.