The United Nations-backed trial of the four most senior surviving members of Cambodia’s murderous Khmer Rouge regime began on Monday, three decades after its “year zero” revolution marked one of the darkest chapters of the 20th century. The defendants, infirm and ranging in age bwteen 79 and 85, were among the inner circle of the late Pol Pot, the French-educated leader of the Khmer Rouge’s ultra-Maoist “Killing Fields” revolution.