When the U.S. and India identified underwater domain awareness technologies for cooperation in India in February, few noticed what lay beneath the phrasing: an implicit acknowledgement that India’s next two nuclear-powered ballistic-missile submarines—INS Aridhaman and its sister vessel S4*—are approaching the operational threshold. This maritime nuclear cooperation is unprecedented: a Russian strategic partner is now fielding its crown-jewel submarine technology alongside the United States, its Cold-War adversary. The boats will complete a four-submarine fleet by 2027, giving New Delhi what strategists describe as a quasi-continuous at-sea deterrent in the Indian Ocean Region. For the world’s nuclear powers, each with submarines patrolling these same waters, this is not a regional story; it is the quiet expansion of nuclear geography.
India’s 7,000-tonne S4-class vessels represent more than technical progress. They transform the Indian Ocean from a zone of deterrence stability into one of latent proliferation and maritime compellence. The logic was visible in 2019, when India conducted its first deterrence patrol amid conflict with Pakistan, and again during the recent May 2025 crisis, when New Delhi deployed its Carrier Strike Group and hinted at undersea readiness. By July, the Defence Minister and Chief of Defence Staff publicly warned that the next conflict would be fought in the maritime domain.
Publicly branded as seventy-five percent indigenous, the programme rests on a composite of imported expertise: an 83 MW pressurised-water reactor based on Russian blueprints; OKBM Afrikantov’s turbine and control systems; Akula- and Delta-IV-derived quieting coatings; and guidance electronics incorporating Western commercial-off-the-shelf modules. Even the sonar chain reflects European exposure—India’s Naval Physical and Oceanographic Laboratory (NPOL) hosted Yves Doisy of Thales Underwater Systems as keynote speaker at its 2018 international sonar conference.
Such transnational engineering would normally provoke proliferation alarms. Yet in India’s case, supplier states have chosen to view the project through a different lens—stabilising vis-à-vis China, benign by design. This selective exceptionalism has eroded the moral centre of the Non-Proliferation Treaty and increased regional strategic instability. If nuclear-propulsion and missile-integration transfers can be sanitised for one state, Kenneth Waltz’s realist provocation—more may be better—acquires unsettling relevance. Either the rules apply to all, or the system concedes that controlled diffusion of nuclear capability is tolerable when politically convenient.
For Pakistan, this diffusion creates a structural asymmetry. The S4-class SSBNs provide India with a survivable second-strike shield while enabling risk-acceptant behaviour at the conventional level—coercive patrols and sea-launched precision strikes conducted under nuclear cover. Once ballistic-missile defence, dual-capable delivery systems, and other disruptive technologies are folded into the mix, India becomes the textbook case of a nuclear power drifting from credible minimum deterrence toward pre-emption and compellence. Deterrence thus shifts from stability by fear to instability by confidence. The consequences extend beyond South Asia: China’s eastern seaboard now lies within the K-4 SLBM’s 3,500-kilometre envelope, compelling the People’s Liberation Army Navy (PLAN) to extend anti-submarine patrols deep into the Bay of Bengal. Every kilometre of new reach compresses another power’s sense of sanctuary.
The Western response has been paradoxical. In Washington and Canberra, India’s sea-based deterrent is treated as a net contribution to Indo-Pacific stability, even as it intertwines with COMCASA, BECA, and QUAD data-sharing frameworks that could implicate allied intelligence, surveillance, and reconnaissance assets in any Indian maritime adventure. A single sonar ping or periscope sighting in the Arabian Sea could draw multiple nuclear navies into a crisis of misattribution, a textbook entanglement dilemma for the twenty-first-century Indo-Pacific.
Moscow, meanwhile, confronts a quieter reckoning. The technologies it transferred under the Russia–India Nuclear (RUIN) Submarine Deal now reside in a fleet increasingly interoperable with Western partners. Russia’s tolerance of Indian collaboration with European and American systems may purchase short-term goodwill in New Delhi, but it undermines Moscow’s deterrence credibility in the longer run. In effect, Russia has midwifed a platform whose strategic loyalty is drifting Westward.
If proliferation through partnership becomes the accepted norm, the Indian Ocean’s undersea domain will soon resemble cyberspace—an unregulated commons of latent deterrence. By 2027, India will also operate SSBNs besides those of U.S., Russia, China, UK, and France in these waters without a single binding code of conduct. The Cold War’s tacit no-tailing understanding, mutual avoidance of stalking one another’s SSBNs, has no regional equivalent. The next accidental collision could involve Karachi and Visakhapatnam as easily as Norfolk and Murmansk once did.
The deeper irony is that the more India borrows foreign technology to serve it global power ambition, the more it complicates the deterrence stability in the Indian Ocean Region. The world thus faces a choice: either restore universality to technology-transfer norms or admit that selective diffusion, if permissible for one democracy, cannot be denied to others seeking balance. If some nuclear weapons make wars less likely, denying that logic to some while enabling it for others is not realism; it is privilege.
India’s maritime nuclear expansion, therefore, is more than an Indian achievement. It is a watershed in the global management of nuclear legitimacy. Whether the Indian Ocean becomes a theatre of restraint or a laboratory of exceptions will depend on how the major powers, those whose submarines glide silently beneath its surface, choose to read this moment: as a warning, or as a precedent.
Author: Dr Zahir Kazmi is an Arms Control Advisor to the Strategic Plans Division, National Command Authority, Pakistan, and a former brigadier. The views expressed are his own.
