A glance at Colombia's half-century guerrilla conflict:

HOW IT STARTED

The 1948 assassination of populist firebrand Jorge Eliecer Gaitan led to a political bloodletting known as "La Violencia." Tens of thousands died and groups of peasants joined with communists to arm themselves against encroachments by the state. A 1964 military attack on their main encampment led to creation of the Revolutionary Armed Forces of Colombia, or FARC.

REBEL AIMS

Though nominally Marxist at its founding, the FARC's ideology has never been well defined. It has sought to make Colombia's conservative oligarchy share power, with land reform a priority in a country where more than 5 million people have been forcibly displaced by far-right militias in the service of wealthy ranchers, businessmen and drug traffickers. The FARC lost popularity over the years as the group began to rely financially on kidnapping, extortion and taxes on cocaine production and illegal gold mining.

US INVOLVEMENT

Washington's support for the military in the 1980s and '90s helped Colombia defeat the Medellin cocaine cartel. But a drug corruption scandal involving the ascendant Cali cartel weakened the government and the FARC battered the state. In 2000, the U.S. instituted Plan Colombia, dedicating billions to counterdrug and counterinsurgency efforts that helped weaken the FARC and kill several top commanders.

THE CONFLICT AND THE COST

Colombia's conflict has claimed more than 220,000 lives since 1958, most of them civilians. In the past two decades, much of the killings were inflicted by far-right militias that made peace in 2003 with President Alvaro Uribe's government. The FARC's most prominent victims were kidnapped ranchers, politicians and soldiers, held in jungle prisons for as long as 12 years. Its most famous captives were presidential candidate Ingrid Betancourt and three U.S. military contractors who were rescued in 2008.

PEACE EFFORTS

A peace pact with rebels in the mid-1980s collapsed after death squads killed at least 3,000 members of the FARC's political wing. Another effort fell apart in 2002 after the rebels hijacked an airliner to kidnap a senator. President Juan Manuel Santos renewed talks with the FARC in 2012 in Norway, then in Cuba.

AGREEMENTS SO FAR

The two sides have reached agreements on land reform, political participation of the guerrillas and a joint commitment to combatting the drug trade. The most complicated issue has been how to punish war criminals on both sides of the conflict.

WHAT REMAINS

Santos says he'll put any peace deal before Colombians in a referendum and Congress must approve it. Another, smaller rebel group was not part of the peace deal but has expressed a desire to end its conflict.