For the first time, Colombia will have a leftist president. Gustavo Petro, a former rebel and a longtime senator who has pledged to transform the country’s economic system, has won Sunday’s election, according to preliminary results, setting the third largest nation in Latin America on a radically new path. Mr. Petro received more than 50 percent of the vote, with more than 99 percent counted Sunday evening. His opponent, Rodolfo Hernández, a construction magnate who had energized the country with a scorched-earth anti-corruption platform, just over 47 percent. Shortly after the vote, Mr. Hernández conceded to Mr. Petro. “Colombians, today...