Earlier this year, India’s first indigenously built nuclear submarine quietly pushed out of its base for sea trials, its 6,000-tonne, 111-metre bulk powered by an 83-megawatt uranium reactor. The submarine is capable of lurking effectively undetectable at depth almost indefinitely, as long as there is food for its 110-man crew. In early 2015, if all goes well, INS Arihant will get the nuclear missiles it is designed to carry.

India will join a club of just six nations with nuclear submarines carrying ballistic missiles — and a doctrinal headache.

For more than a decade now, India has kept warheads separate from the missiles that carry them, in an effort to prevent accidents. In times of crisis — like the 2001-02 standoff with Pakistan — delivery platforms and warheads have been brought together, but by some accounts, even then, they were not mated or joined together for delivery.

Prime Minister Narendra Modi was briefed on classified reports calling for a full-time four-star General to take charge of India’s nuclear arsenal — and the case of Arihant explains why.

Last week, Mr. Modi received the most secret briefing he would get — on his role as head of the Nuclear Command Authority, which is empowered to order the nuclear missiles on the Arihant, along with other weapons in the strategic arsenal, to be fired. Mr. Modi, government sources say, was briefed on progress in the submarine tests, as well as the status of the missiles that will arm it.

In March, the Defence Research and Development Organisation conducted the first test of the K-4 missile —capable of delivering a two-tonne nuclear warhead on targets up to 3,000 kilometres away.

Fitted four apiece on to the three nuclear submarines India plans to operate, K-4 will ensure that the country has what experts call an assured second-strike capability — the capacity to ensure retaliation even if the rest of the arsenal is wiped out in a surprise first-strike.

India’s nuclear arsenal, as that of Pakistan, has been physically separated from the delivery platforms — the missiles controlled by the Army, and soon the Navy, as well as the Air Force’s combat jets. The logic is simple: keeping warheads and missiles apart reduces the risks of accidents or unauthorised use.

“For obvious reasons,” says Arun Vishwanathan, a leading nuclear-weapons expert at the National Institute of Advanced Studies, Bangalore, “a nuclear submarine is going to have to carry warheads as well as missiles. This raises significant issues of control, which need to be worked out.”

In addition, nuclear submarines can lose contact with their bases — and officers must decide if this has happened because of technical problems, or because their nation has been obliterated. In 1961, the Soviet submarine B-59, believing that war had broken out, almost fired a 10-kilotonne warhead at the U.S. Flotilla; sub-commander Vasili Arkhipov, one of three officers who had to consent to the decision, alone demurred — averting a nuclear apocalypse.