BOGOTA, Colombia — Colombia’s largest rebel group on Friday called off a unilateral cease-fire in reaction to a military raid on a guerrilla camp that killed 26 of its fighters, further straining negotiations to end the country’s half-century-old conflict. The Thursday attack on the guerrilla camp in Cauca province, which President Juan Manuel Santos called a major blow against FARC, appears to have been in retaliation for the rebels’ own stealth raid last month, also in Cauca, on an army patrol. Ten soldiers were killed in that attack, which led Santos to scrap his own confidence-building gesture: a ban on launching air raids against guerrilla camps. While the FARC negotiators’ declaration of a unilateral truce in December pleasantly surprised many Colombians, it was never fully honored by the estimated 7,000 fighters on the battlefield, many of whom are isolated and on the ropes after more than a decade-long, U.S.-backed offensive.