The meaning of Gustavo Petro’s victory in Colombia

When South America experienced the so-called “Pink Tide” in the 2000s, with left and center-left presidents winning elections in multiple countries, Colombia stood as the most notable outlier. The country was controlled by right-wing leader Álvaro Uribe and his allies, a grip on power that lasted for more than two decades. Before that, liberals and conservatives had been taking turns in power for more than a century.

After years of rising anti-establishment sentiment, Colombia made history on Sunday by electing leftist former Bogotá mayor Gustavo Petro as its new president. 

Mr. Petro beat populist newcomer Rodolfo Hernández by a tight margin. With 99.9 percent of ballots counted, Mr. Petro edged Mr. Hernández 50.4-47.3. Turnout reached 58 percent, the highest since 1998.

“We are writing a new history for Colombia, for Latin America, and for the world. We will not betray our electorate, which has given us a screaming call for change,” Mr. Petro said in his victory speech.

“Many people fought and died, or are currently in jail and treated as criminals, just because they believed in change. (…) We will build a new life-affirming Colombia, based on peace, social justice and climate justice,” the winning candidate added.

Mr. Petro has had a long career in Colombian politics. He started out as an intellectual leader in the left-wing M-19 guerrilla in the 1980s, before becoming a key broker in Colombia’s peace agreements and constitutional rewrite in the 1990s. Since then, he became a prominent opposition leader and served as mayor of the capital city in the 2010s.

As the Uribe-led right took near full control of the country, becoming the steadiest U.S. ally in the region and garnering strong backing from powerful military and paramilitary groups, Mr. Petro was the figurehead for those looking to push back against those in power. 

He questioned the government’s repeated violations of peace agreements, extrajudicial killings, and ties with the criminal underworld, as well as its role in consolidating an economic elite in a highly-unequal country.

Despite initially being seen by large swathes of the electorate as a key factor in the relative pacification of Colombia after years of mass-scale political violence, Mr. Uribe and his allies progressively lost credibility, with incumbent Iván Duque seen unfavorably by three out of four Colombians in recent surveys, and protest rallies led by the left growing in size and popularity.

Messrs. Petro and Hernández campaigned on platforms of change and put constant emphasis on criticizing the outgoing administration, backing peace agreements, and calling for the normalization of diplomatic...