Rubén Sánchez, an army corporal, squints at the distant horizon of Colombia's Andean highlands for any sign of movement. Clouds swirl over the nearby peaks. The thud of a Black Hawk helicopter fades into silence.

On patrol in Sumapaz, a bleak, 3,500m-high mountain pass south of Bogotá, the capital, Cpl Sánchez feels that his part of the war against the rebels from the Revolutionary Armed Forces of Colombia (Farc) is being won.