The spread of long-range ballistic missiles took a step forward on May 7 with India's successful flight test of its Agni-III missile that can carry a nuclear warhead as far as Beijing. But the difference between this and other missile developments is that India's missiles — like those of the United States, Britain, France and Israel — are not used to threaten others and instead help deter potential aggressors. With nuclear missile-armed neighbors like China, Russia and Pakistan, India needs an effective deterrent. But for years New Delhi concentrated on developing tactical missiles to deter Pakistan, which India fought three...