The paper examines the nuclear arms race in South Asia. The security competition between India and Pakistan has been characterised by an action-reaction spiral. The paper traces the nuclear arms competition between the two South Asian rivals and argues that this relationship is a classic case of a security dilemma whereby one state’s actions cause insecurity to the other, and the other state’s efforts to augment its security make the first one insecure. The paper, thus, develops a theoretical explanation for the incessant arms race between India and Pakistan. India decided to take the nuclear weapons route, Pakistan followed; India developed sophisticated ballistic and cruise missiles, Pakistan followed; India adopted an aggressive doctrine, the Cold Start, Pakistan responded by developing tactical nuclear weapons; India is developing ballistic missile defence and Pakistan is sure to respond by adjusting its nuclear force posture. It is thus not the individual weapons systems but the security dilemma that is the cause of instability in highly volatile South Asia.