Last month, Colombians went to the polls to elect the successor to President Juan Manuel Santos. The unpopular and Nobel Prize-winning outgoing president was the architect of a 2016 peace deal with the Revolutionary Armed Forces of Colombia (the FARC), ending five decades of civil war.

On June 17, after months of bitter and sometimes violent campaigning among candidates across the political spectrum, two men will advance to a runoff: the right-wing Iván Duque of the Democratic Center party and left-wing Gustavo Petro of Humane Colombia.

This is a deeply divisive election

In a country still healing from decades of violence, this election pits the hard right and left against each other. Duque vehemently opposes the peace deal, which raises concerns about the implementation process and the long-term success of the deal itself. Petro is a former left-wing guerrilla from a group known as the April 19th Movement, or M-19.

Petro has amassed years of government experience since his guerrilla days. He served as a congressman and senator, and from 2011 to 2014 was the highly controversial mayor of Bogotá, Colombia’s capital. Voters recalled him after his attempts to reform municipal trash and recycling collections left tons of garbage on the streets.

Four years later, Petro’s anti-establishment platform focuses on battling corruption and inequality, protecting the environment, and promoting land and energy sector reform. His redistributive platform invites comparisons to Venezuela’s former leader, Hugo Chávez.

Should Petro win, he would be Colombia’s first leftist president. The FARC, meanwhile, will have 10 guaranteed seats in Colombia’s next Congress, part of the guerrilla group’s transformation into a political party — as detailed in the peace plan.

With the country’s left-wing leaders on the rise, here are five things to know about the M-19:

1. M-19 drew its support from urban intellectuals

The M-19 formed to protest the April 19, 1970, presidential election, in which alleged campaign fraud gave a victory to conservative candidate Misael Pastrana Borrero over populist favorite Gustavo Rojas Pinilla. Unlike Colombia’s other rebel groups, the M-19 garnered a significant amount of early support from the left-leaning intellectual elite.

This was an urban guerrilla movement, drawing leaders and members from wealthy families and organizations. The group’s efforts to understand working class struggles and their “Robin Hood-like” deeds won them significant popular support in Colombia’s urban centers.

2. They showed how hostage taking can succeed

In 1980, the M-19 staged a dramatic hostage taking at the Embassy of the Dominican Republic in Bogotá. A group of 16 heavily armed guerrillas stormed the embassy and captured dozens of top foreign diplomats attending a reception. In exchange for their safe return, the M-19 demanded the release of 311 political prisoners — including the group’s founder, Jaime Bateman Cayón — as well as a $50 million ransom and the publication of the M-19 manifesto in the major newspapers of all the hostages’ countries.

When the diplomatic hostages were released two months later, both sides claimed a victory. The M-19 gained significant international publicity and a rumored $2.5 million ransom. The Colombian government granted neither political concessions nor prisoner releases — once seen as worse than material support through ransom payments.

Along with the 1979 takeover of the U.S. Embassy in Iran, this attack ushered in the heyday of embassy sieges, from the Iranian Embassy in London to the Japanese Embassy in Lima, Peru. In the 1980s, the M-19 committed the bulk of kidnappings in Colombia. The better-known FARC and National Liberation Army (ELN) would not take hostages on this scale for another decade, after the M-19 demobilized.

3. … and how it can fail

In 1985, the M-19 launched one of the deadliest attacks in Colombia’s violent history. Frustrated with the military’s violation of their 1984 cease-fire agreement, the group lay siege to the Colombian Palace of Justice (the country’s highest court), to coerce the justices to bring then-President Belisario Betancur to trial. Unlike the successful embassy negotiations, the Betancur government refused to bargain with the hostage-takers, instead returning demands with fire.

During the 27-hour ordeal, 11 justices and 55 courthouse employees were among the more than 100 people killed. The M-19 achieved none of their demands and lost at least 30 of its own. Controversy surrounds the attack, including a supposed link to Medellin Cartel boss Pablo Escobar (who allegedly paid the guerrillas to destroy criminal records), and the forced disappearance of 11 attack survivors accused of aiding the guerrillas.

4. The M-19 sparked the creation of Colombia’s right-wing paramilitaries

In 1981, the group kidnapped Martha Nieves Ochoa, the sister of prominent members of the powerful Medellin drug cartel. Rather than pay the ransom demand, the cartel sought revenge, collaborating with other gangs to form the Death to Kidnappers (MAS) group, a right-wing paramilitary organization targeting the M-19 and others on the left. Paramilitary violence then spread widely across Colombia, resulting in more than 24,000 murders and thousands of forced disappearances in Colombia’s long civil war.

5. They helped pave the road from guerrilla to government

The M-19 became the first armed group to sign a peace deal in Colombia, almost 30 years ago. Although nearly 7,000 combatants put down their arms, the violence continued. Hundreds of M-19 ex-combatants were assassinated by hit men and paramilitaries. Former M-19 leader and presidential aspirant Carlos Pizarro Leon Gomez was one of the three presidential candidates killed in the months before the 1990 election.

Violent transitions were a regular feature of Colombian demobilizations. In the 1980s, when the FARC first attempted a peaceful transition to politics, right-wing paramilitaries assassinated thousands of its Patriotic Union candidates. These events loom over current negotiations, as the FARC demand security guarantees for ex-combatants targeted throughout the campaign.

Yet the M-19’s agreement laid important foundations for transition and success. First, these and other narrow negotiations ushered in a wave of peace talks over the last 30 years, with dozens of paramilitary organizations, the FARC and now the ELN coming to the negotiating table for far more complex efforts at disarmament, demobilization and reintegration. Second, the M-19 agreement helped launch major reforms to the Colombian Constitution, enshrining protections for ethnic minorities, facilitating broader participation and reforming the country’s justice system.

And this agreement also launched the political careers of former rebel reformers, one of whom may be one step away from the Colombian presidency.

Danielle Gilbert is a 2018-2019 Minerva/Jennings Randolph Peace Scholar at the United States Institute of Peace and a PhD candidate in political science at George Washington University.