MITÚ, COLOMBIA—Alberto Diaz, 14, a Marxist guerrilla fighter in the Colombian civil war, is 25 years younger than the war itself. "President Arango and his corrupt right-wing regime must fall," said the pubescent Diaz, whose rebel group, the Revolutionary Armed Forces of Colombia, has been trying to topple the government since the early 1960s. "This has been my dream ever since 1999, when I was just an 11-year-old child." Diaz then popped a pimple on his chin and wiped the pus on the barrel of his AK-47.