Published on: Dec 25, 2025

India has moved a decisive step closer to completing its sea-based nuclear deterrent with the successful test-firing of the K-4 submarine-launched ballistic missile (SLBM), a critical component of the country’s nuclear triad. The missile was tested by the Defence Research and Development Organisation (DRDO) on December 23 from INS Arighaat, India’s second indigenously built nuclear-powered ballistic missile submarine (SSBN), in the Bay of Bengal.

With a strike range of around 3,500 kilometres, the K-4 missile represents a major leap in India’s undersea nuclear capability. Although a few additional test-firings are still required before its formal induction, defence officials indicate that the programme is now in its final phase of operational validation. Once inducted, the K-4 will be operated by the tri-services Strategic Forces Command (SFC), marking a near-complete realisation of India’s long-cherished goal of a credible, survivable sea-based nuclear deterrent.

A Crucial Milestone in India’s Nuclear Doctrine

India’s nuclear doctrine is anchored in the principle of “no first use” and credible minimum deterrence. For such a doctrine to be effective, the country must retain assured second-strike capability—the ability to respond with nuclear force even after absorbing a first strike. Sea-based nuclear assets, particularly SSBNs armed with long-range ballistic missiles, are widely considered the most survivable leg of a nuclear triad.

The successful launch of the K-4 missile from a submerged SSBN platform reinforces India’s ability to deploy nuclear weapons covertly and sustain deterrence in an increasingly contested maritime environment. Unlike land-based missiles or air-delivered nuclear weapons, submarines operating deep underwater are extremely difficult to detect and neutralise, making them central to long-term strategic stability.

Delays and Regional Sensitivities

While the Indian government typically maintains strict silence on nuclear weapons tests, it has emerged that the K-4 launch was delayed from its earlier window of December 1–3. The postponement was reportedly due to the presence of a Chinese Ocean Mineral Resources Research Vessel near the southern edge of the designated navigation area, approximately 115 nautical miles away, on December 3.

Although the Chinese vessel was not equipped with missile tracking systems, its presence highlighted the heightened surveillance and strategic competition unfolding in the Indian Ocean Region (IOR). In recent weeks, several Chinese research and survey ships—including Shi Yan 6, Shen Yi Hao, and Lan Hai—have been active in the region. Additionally, elements of China’s 48th anti-piracy escort force, including a Luyang III-class destroyer, a Jiangkai II-class frigate, and a Fuchi-class tanker, were operating near the Gulf of Aden.

These developments underscore the complex maritime security environment in which India is advancing its undersea deterrent capabilities.

The Arihant-Class SSBN Programme

The K-4 missile is designed to be the primary armament of India’s Arihant-class SSBNs. While the lead submarine, INS Arihant, is equipped with shorter-range K-15 missiles (with a range of about 750 kilometres), all subsequent boats in the class are intended to carry the longer-range K-4.

INS Arighaat, from which the recent test was conducted, is the second submarine in this class. The third SSBN, INS Aridhaman, is expected to be commissioned in the first quarter of 2026, further strengthening India’s continuous at-sea deterrence posture. On October 16 this year, Defence Minister Rajnath Singh also launched S4★ (S4 Star), the fourth and final SSBN of the Arihant class. This submarine is expected to be inducted into the Strategic Forces Command towards the end of the decade.

Together, these platforms will provide India with the ability to keep at least one nuclear-armed submarine on patrol at all times—a key requirement for credible deterrence.

Looking Ahead: K-5 and the Next Generation

Beyond the Arihant class, India is already planning its next generation of SSBNs. These future submarines are expected to displace around 10,000 tonnes, significantly larger than the current Arihant-class boats, and will be armed with the K-5 ballistic missile. The K-5 is projected to have a range exceeding 5,000 kilometres, enabling India to target distant adversaries without having to deploy submarines close to hostile shores.

Such capabilities would further enhance India’s strategic reach and survivability, especially as regional missile defence systems and surveillance networks continue to evolve.

Strengthening the Attack Submarine Fleet

Parallel to its SSBN programme, India is also expanding its nuclear-powered attack submarine (SSN) capabilities. Unlike SSBNs, which are primarily strategic assets, SSNs are designed for offensive and defensive roles, including hunting enemy submarines, escorting carrier groups, and projecting power in contested waters.

India has already signed an agreement with Russia to lease an Akula-class nuclear-powered, conventionally armed attack submarine by 2028. Reports suggest that Russia has also offered a second SSN on lease. In a significant boost to indigenous capability, the Modi government cleared the Indian Navy’s proposal on October 9 to build two SSNs domestically under the “Make in India” initiative.

SSNs offer major advantages over diesel-electric submarines (SSKs), including virtually unlimited range and endurance, higher speeds, and greater payload capacity for torpedoes and missiles. While SSKs are quieter and can be highly effective in littoral waters, SSNs and SSBNs are indispensable for sustained blue-water operations and strategic deterrence.

Growing Maritime Threats

India’s push to strengthen its undersea forces comes against the backdrop of an increasingly challenging security environment. China and Turkey are supplying advanced guided-missile frigates to Pakistan, while Turkey is upgrading Pakistan’s existing submarine fleet and China is delivering new Yuan-class submarines to Islamabad.

At the same time, China’s naval expansion continues at a rapid pace. The commissioning of the Fujian, China’s newest and most advanced aircraft carrier, is expected to enable regular carrier-led patrols in the Indian Ocean from 2026 onwards. Chinese carrier-based aviation has demonstrated rapid learning and operational sophistication, posing a growing challenge in the broader Indo-Pacific region.

In such a scenario, India faces strategic pressure both in the Arabian Sea and across the wider Indian Ocean Region. Defence planners increasingly view nuclear-powered submarines—both SSBNs and SSNs—as the most effective counter to surface threats, given their stealth, endurance, and survivability.

A Sea-Based Deterrent Comes of Age

The successful K-4 missile test marks more than just a technical achievement; it signals the maturation of India’s sea-based nuclear deterrent. With multiple SSBNs nearing operational readiness, longer-range missiles in the pipeline, and a growing attack submarine fleet, India is steadily building a robust undersea force capable of safeguarding its strategic interests.

As geopolitical competition intensifies in the Indo-Pacific, the credibility of India’s second-strike capability will play a central role in maintaining deterrence and strategic stability. The K-4 missile’s progress suggests that India is now closer than ever to achieving that objective beneath the waves.