India’s evolving submarine strategy has brought the Indian Ocean into the forefront of South Asia’s nuclear balance, unsettling Pakistan’s defence establishment. With the advent of Continuous at Sea Deterrence (CASD), India ensures at least one nuclear-powered ballistic missile submarine (SSBN) remains on patrol, guaranteeing a secure second-strike capability—something that dramatically raises the stakes of any nuclear confrontation.

Since the commissioning of INS Arihant in 2016 and INS Arighat in 2024, India has rapidly scaled up its undersea deterrence by operationalising and expanding the S-4, S-4*, and future S-5 class submarines, with the latter projected to displace over 13,500 tons and carry high-end nuclear missiles.

Central to this deterrence is India’s K-series submarine-launched ballistic missiles (SLBMs). The K-15, with a strike range up to 1,500 km, is already deployed, while the K-4, capable of hitting targets 3,500–4,000 km away, has been repeatedly tested. In parallel, India is developing the MIRV-enabled K-5 with ranges exceeding 5,000 km and the K-6, which reportedly aims for hypersonic capabilities and 8,000 km endurance.

This strategic missile ecosystem makes India’s sea-based deterrence not just survivable but potent enough to hold at risk adversary centres far from Indian shores, including Chinese and Pakistani mainland targets.

Geography grants India added advantage. While the Arabian Sea is crowded and heavily surveilled by Pakistan and Chinese naval assets, the Bay of Bengal offers India’s SSBNs deeper, less-contested waters that facilitate stealth operations.

This is complemented by the underground Project Varsha base near Visakhapatnam, designed to securely berth SSBNs and maintain uninterrupted deterrence patrols away from prying eyes and satellite detection capability. Together, these initiatives are transforming the Bay of Bengal into a secure bastion for India’s nuclear fleet—much like the Soviet “bastion strategy” during the Cold War.

Strategists anticipate India will ultimately build a fleet of 10–12 nuclear submarines, comprising both SSBNs for strategic deterrence and SSNs for escort and hunter-killer operations. Such a fleet provides India with layered options against twin threats posed by Pakistan in the west and China from the east. Pakistan, in turn, finds itself compelled to rethink its own deterrence doctrine.

Analysts in Islamabad argue that India’s “nuclear fortress” posture erodes the credibility of Pakistan’s deterrence and may compel an investment in new nuclear delivery options, expansion of its sea-based deterrence efforts, or significant enhancement of anti-submarine warfare (ASW) capacities, including maritime surveillance drones, advanced sonar networks, and fast-attack submarines.

At the same time, concern lingers in Pakistani defense circles that the growing Indian arsenal could spark an uncontrolled arms race in the region. The scale and sophistication of India’s strategic buildup—particularly its pursuit of hypersonic SLBMs and MIRV technology—might lead Pakistan to press ahead with its own riskier doctrines, raising the chance of miscalculation from both sides. Consequently, some Pakistani analysts stress the urgent need for a bilateral naval hotline—modeled on Cold War-era U.S.-Soviet channels—to reduce the risk of accidental escalation at sea.

The Indian Ocean has thus quickly transcended its role as a global trade hub into a crucial theatre of nuclear rivalry.

For India, CASD represents the assurance of survivability and strength in a nuclear exchange; for Pakistan, it is a strategic provocation that challenges its deterrence calculus on both political and military fronts.

This shifting balance portends a new era of undersea nuclear competition, one in which discreet deployments, survivability, and technological leaps may matter as much as traditional land-based postures.

Comparative table highlighting the India vs Pakistan undersea nuclear capabilities and doctrine differences:

India Vs Pakistan: Undersea Nuclear Capabilities

CategoryIndiaPakistan
Nuclear Submarine Fleet2 commissioned SSBNs (INS Arihant, INS Arighat), S-4/S-4* under trials, S-5 class (13,500 tons) under construction; long-term goal of 10–12 nuclear submarines (SSBNs + SSNs)No operational SSBNs; operates conventional Agosta-90B and Khalid-class diesel-electric submarines; plans for 8 Chinese-origin Hangor-class (Type 039B) by 2030 (non-nuclear, AIP propulsion)
SLBM CapabilityK-15 (750–1,500 km, operational), K-4 (3,500–4,000 km tested), K-5 (>5,000 km, MIRV in development), K-6 (~8,000 km projected with hypersonic capability)No dedicated SLBMs; relies on land-based ballistic missiles (Shaheen, Ababeel) and limited cruise missiles (Babur SLCM with ~450–700 km range, tested on Agosta-90B submarines)
Nuclear Doctrine at SeaContinuous at Sea Deterrence (CASD) with constant SSBN patrols to ensure survivable second-strike capabilityNo established CASD; nuclear posture is primarily land-based with nascent interest in limited sea-based deterrence
Strategic BasesProject Varsha underground base near Visakhapatnam for SSBN shelter, command-and-control, and discreet deploymentsKarachi and Ormara naval bases; no specialized nuclear submarine base
Second-Strike SurvivabilityHigh: stealth SSBN deployments in Bay of Bengal, MIRV capability, increasing missile rangesLow: survivability dependent on air and land-based assets; limited SLCM option vulnerable to detection
ASW (Anti-Submarine Warfare)Growing ASW fleet with P-8I Neptune aircraft, advanced sonar nets, ASW corvettes, and planned SSNs for hunter-killer rolesLimited ASW capability; primarily conventional means with Chinese assistance, but still inferior to India’s surveillance and P-8I fleet
Regional Strategic ImpactMoves toward a “nuclear fortress” in Bay of Bengal; credible and survivable deterrent against both China and PakistanVulnerability to Indian SSBN patrols raises strategic anxiety; reliant on asymmetric strategies and potential escalation in land-based nuclear forces
Future Outlook (2030)Fully operational nuclear triad, with 5–6 SSBNs in service including S-5 class, ensuring uninterrupted CASDMay deploy SLCMs on future Hangor-class submarines; unlikely to match India’s SSBN fleet; focus likely on ASW improvements and tactical nuclear escalation options

By 2035, India is projected to have a robust sea-based nuclear triad with multiple SSBN patrols at all times, ensuring CASD, while Pakistan will likely rely only on short-range SLCMs aboard conventional submarines, which are survivable only in peacetime but vulnerable during conflict. India’s posture trends toward a “Soviet-style bastion strategy,” while Pakistan remains locked into asymmetric responses and tactical nuclear options.

IDN (With Agency Inputs)