The FARC is the oldest guerrilla group operating in the Western Hemisphere and began in 1964 as a peasant insurgency with leftist aims. The group – which still draws largely from the rural poor and continues to claim political aspirations – has been criticized in recent years for turning toward trafficking drugs and carrying out kidnappings to finance its war against the government.
The FARC grew to 19,000 members in the 1990s, but was weakened by military offensives launched by former President Álvaro Uribe with help from the United States under Plan Colombia, which began in 2000. Today there are an estimated 9,000 fighters.
Mr. Santos has continued to fight the FARC, which the US and the European Union consider a terrorist organization. Though top FARC leaders have been captured or killed and thousands of members have demobilized, it is still a deadly force in Colombia.