A new submarine promises to give the world’s most populous democratic nation a powerful second-strike nuclear capability. The INS Arihant, India’s first nuclear ballistic-missile submarine, will finally give the country nuclear weapons that could survive a surprise first strike and go on to deal a crushing retaliatory blow to the enemy. The new sub will complete India’s triad of air, land and sea nuclear forces.

India tested its first weapon, an eight-kiloton device nicknamed Smiling Buddha, in 1974. Although small in yield, the device was a remarkable technological achievement that thrust the young country into the exclusive, so-called “nuclear club” that had until then consisted of the United States, Soviet Union, United Kingdom, France and China.

India is believed to have 520 kilograms of plutonium—enough for, according to the Arms Control Association, “100 to 130 warheads.” New Delhi describes this a “credible minimum deterrent” against neighboring nuclear powers China and Pakistan. India has a firm No First Use policy with regard to nuclear weapons, vowing to never be the first to use them in any conflict and only use them to retaliate in kind.

Nuclear-armed submarines are an ideal basing solution for a country such as India. While less accurate than land-based missiles and less flexible than air-launched weapons, ballistic-missile submarines are the most difficult to destroy in a first strike. Hiding in the vastness of the oceans, a nuclear-armed submarine is nearly invulnerable. And, in the logic of nuclear deterrence strategy, an invulnerable nuclear arsenal makes for an invulnerable country.

The Arihant program goes back more than three decades, to the vaguely named Advanced Technology Vessel. Begun in 1974, ATV was broadly conceived as a project to research nuclear propulsion and, down the road, field a indigenously developed and constructed nuclear-powered submarine. The program was a collaboration between the Bhabha Atomic Research Centre, the Indian Navy and the Indian government’s Defence Research Development Centre.

By 1995, ship-sized reactor trials were underway at the Bhabha Centre in Mumbai. According to Combat Ships of the World, the reactor had been under development since 1985, weighed 600 tons and was “entirely unsuccessful.” By 1989, Russian nuclear scientists and engineers joined the project, and yet the program still failed to yield a viable reactor. In 1998, the Indian government threw in the towel and purchased a reactor design outright from Russia, and by 2004, a working eighty-megawatt prototype reactor had been built, tested and achieved criticality.

Hull began construction in 1998 at Visakhapatnam, but could not be completed due to the lack of a working reactor. The hull itself is variously reported as based on the Russian Akula/Project 971–class nuclear attack submarine or the ex-Soviet Charlie II class. Combat Fleets of the World claims it is based on the Akula, and lengthened an additional thirty feet to accommodate a missile compartment. Other sources claim it is based on the Charlie II class, one of which was leased to India from 1988 to 1991 and served as INS Chakra. At either rate, the submarine is estimated to be 330 to 360 feet long, with submerged displacement of 6,500 tons. It is the smallest ballistic-missile submarine in the world, with the possible exception of the North Korean Gorae class.

Thanks to nuclear propulsion, Arihant can do twelve to fifteen knots on the surface and twenty-four knots underwater. Maximum diving depth is unknown, and probably a closely held secret, but the Akula class is known to dive to six hundred meters. The submarine is manned by a crew of ninety-five to one hundred.

Arihant was officially launched in 2009. The onboard reactor reached criticality in 2013, and the ship began sea trials in late 2014. It was officially commissioned into service in August 2016. According to Naval Technology, the total price tag was $2.9 billion.

Arihant’s name literally translates to “Slayer of Enemies,” and the ship’s armament makes it the greatest concentration of firepower in Indian history. The submarine was built with four missile tubes mounted in a hump behind the conning tower. The four can carry twelve K-15 Sagarika (“Oceanic”) short-range ballistic missiles. K-15 has a maximum range of just 434 miles, making it capable of hitting just the southern half of Pakistan.

Alternately, the sub can carry four K-4 medium-range ballistic missiles with a 2,174-mile range, capable of hitting targets as far away as Beijing. Both the K-4 and the K-15 are nuclear capable, but the warhead yield is unknown. India has yet to master multiple independently targetable reentry vehicle (MIRV) technology, so whatever the yield of the warhead, K-4 and K-15 carry just one of them.

In order to be credible, a seagoing nuclear deterrent must have at least one submarine on patrol at all times. The second ship in class, Aridhaman, is under construction in Visakhapatnam, and India plans to have as many as four boomers by 2020—the same number as the United Kingdom and France. With the four nuclear-armed boats completed, India may finally achieve its goal of strategic invulnerability.

Kyle Mizokami is a defense and national-security writer based in San Francisco who has appeared in the Diplomat, Foreign Policy, War is Boring and the Daily Beast. In 2009 he cofounded the defense and security blog Japan Security Watch. You can follow him on Twitter: @KyleMizokami.

Image: A drawing of the INS Arihant nuclear submarine. Wikimedia Commons/gagan@BRF