India has what is known as a full nuclear "triad" - the ability to deliver nuclear bombs from the ground, air and sea. Its ground-based ballistic missile, the Agni-III, has an approximate range of 3220km.
Its cruise missile - called Brahmos, developed jointly with Russia - can be launched from land, sea or air. Indian Air Force fighters have been modified to deliver nuclear bombs. India also has a 6000 tonne indigenously developed ballistic missile submarine called the Arihant.
Although Pakistan is a much smaller country, it has a slightly larger arsenal - although it has less capacity than India to deliver such weapons. Pakistan had approximately 140 nuclear warheads in 2017, up from 120 to 130 a year earlier.
Unlike India, Pakistan's ballistic missile range is only 1930km, although there are powerful versions under development. It does not have any nuclear-equipped submarines.
However, Pakistan has more plutonium-production reactors than India and the capability to produce up to 20 nuclear warheads a year. The country's defence expenditure was more than 16 per cent of national expenditures in 2017, far higher than India's 9 per cent.
Experts say that Pakistan's focus on developing short-range nuclear-capable missiles is part of a tactical mission to be able to counter India's superior strength in ground-based armed forces.
At the height of the US-Iran nuclear deal negotiations in 2015, a New York Times editorial said the "biggest concern" was that Pakistan had the "world's fastest-growing nuclear arsenal."
Neither country is a signatory to the Non-Proliferation Treaty of 1968, which seeks to prevent countries from developing nuclear weapons. The United States, China, Britain, France and Russia are the five declared nuclear weapons states under the treaty - those that have built and tested nuclear explosives before 1967.
Over the years, India has maintained a nuclear no-first-use policy, stating that the country will not be the first to initiate a nuclear strike and that its weapons would be used only for retaliation. But the recent expansion of its arsenal has signalled a possible shift in this doctrine.
Bharat Karnad, an Indian national security expert, has said that India's doctrine could function differently in reality than on paper. In case of a contingency, weapons would be launched on "warning" - that is, on the detection of a signal from Pakistan, for instance - or "launched on launch," if the other side were to initiate a nuclear strike.
Pakistan's policy has emphasised "full-spectrum deterrence," which essentially means competing with India to "deter the kind of activities we have seen over the last few days," said Toby Dalton, co-director of the Nuclear Policy Programme at the Carnegie Endowment for International Peace.
For security experts, the current clash between India and Pakistan is fascinating, if alarming. "This is like reality is playing out and testing deterrence theory," said Uday Bhaskar, director of the Society for Policy Studies and a security expert in New Delhi.
Dalton expressed concern that domestic political considerations may tempt leaders to escalate the crisis. "The probability may be 1 per cent, but the consequences are so terrible," he said.
