Tensions between India and Pakistan cause deep concern internationally, largely because both countries have nuclear arsenals.
Both are also relatively new nuclear powers and there are concerns that neither fully understands the nuclear doctrine of the other.
India and Pakistan stunned the world with back-to-back nuclear tests in May 1998, sparking fears of an arms race on the sub-continent.
Unlike Russia and the United States, they are not bound by any treaty obliging them to reveal the extent of their arsenals. And perhaps most worryingly, no one knows for sure how many warheads each country has.
Experts fear that India and Pakistan do not have the mutual understanding that was key to the nuclear standoff during the Cold War
Neither India nor Pakistan appear to have enough nuclear weapons or accurate missiles to carry out effective strikes on each other's military forces, a 2002 report from the Center for Strategic and International Studies (CSIS) said.
This could prompt either side to switch attention to major cities - with potentially devastating consequences, the report suggested.
In a separate study by NewScientist.com, nuclear researchers warned that if 10% of the estimated arsenal of both countries is used on 10 of their largest cities, at least 3 million people would be killed and 1.5 million injured.
In military terms, India remains considerably stronger than Pakistan. This is the strategic reason why the government in Islamabad has developed a nuclear deterrent - to defend itself against its more powerful neighbour.
Prithvi I: 200km
Prithvi II: 250km
Agni II: 2,000km
Estimates of the number of warheads belonging to each side vary.
In 2002, defence experts at Jane's speculated that Pakistan could have as many as 150 warheads against an Indian arsenal of 200 to 250.
However the Washington-based Centre for Strategic and International Studies provides a lower estimate, suggesting that India has about 60 nuclear warheads.
The Federation of American Scientists suggests that Pakistan possesses about 25 warheads.
The majority of these would be designed for delivery by bombs, not missiles, and it is unclear if the weapons are fully assembled or if their component parts are stored separately.
India's nuclear weapons programme is clearly not unrelated to developments in Pakistan. But it is not a simple two-way race.
In large part, it is an effort to secure India's position as an emerging regional player against China, which already has a well-established and capable nuclear arsenal.
China, estimated to have 300 to 600 nuclear warheads, is a prime consideration behind India's weapons programmes.
The fact that Pakistan's weapons programme has had considerable Chinese assistance has only increased India's concerns.
Both countries could probably deliver a nuclear bomb using aircraft.
Hatf III: 300km
Shaheen I: 750km
Ghauri II: 2,000km
Shaheen II: 2,000km
Ghauri III: 3,000km
But the nuclear race has spawned a parallel missile race as each country seeks to develop medium and long-range missiles capable of carrying a nuclear warhead.
Although India and Pakistan have said they do not intend to conduct further nuclear tests they carry out missile tests on a regular basis.
Security experts are uncertain if either country actually has a warhead suitable for delivery by a missile.
The range of their operational missile systems makes them vulnerable to pre-emptive attack once deployed.
India has already test-fired its Agni II missile. This has a range of 2,000km, meaning it could hit anywhere in Pakistan and worry China too.
India is also reported to be working on a 5,000km-range missile and has also suggested it could make a neutron and a hydrogen bomb.
Pakistan has test-launched its Shaheen I and Ghauri I and II missiles, the latter with an estimated range of 2,000km.
Work is in progress on a Ghauri III, which is believed to have a 3,000km range.
Test Ban Treaty
The 1998 nuclear tests were widely condemned by the international community and resulted in US sanctions.
But despite strong pressure from Washington, neither country has signed the Comprehensive Test Ban Treaty.
Nor have they signed the Nuclear Non-Proliferation Treaty (NPT), which obliges the nuclear powers never to transfer their nuclear technology to other countries.
According to the BBC's defence correspondent, Jonathan Marcus, experts are most worried by the lack of sophistication in both countries' nuclear doctrines.
They also fear that they probably do not have the mutual understanding that was so key to the nuclear standoff between the Cold War superpowers.
Our correspondent says deterrence became a sort of nuclear theology - a complex array of doctrines and technological safeguards that made nuclear accident or miscalculation far less likely.
That is not the case between India and Pakistan, however responsible each country may be.