DW: How would you describe the nature of Sino-Pakistani relations?

Andrew Small: Although much of the language used to describe ties between the two sides can sound hyperbolic – "all-weather friends," "deeper than the deepest ocean," "sweeter than honey" – it reflects the unusual nature of a secretive relationship that is the only real friendship that either side has. Founded on a shared enmity for India, the relationship has in some respects run deeper than formal alliances, especially when it comes to nuclear cooperation, and has been remarkably resilient over the dramatic economic and geopolitical shifts of the last few decades.

China and Pakistan have never been treaty allies, do not share any cultural reference points or values, and even their militaries come from radically different traditions. Recent years have seen tensions over matters such as Uighur militants in Pakistan, and Chinese fears about Pakistan's "Islamization."

Yet the deep-rooted strategic alignment remains strong, and all the more so as the stabilization of China's western periphery becomes more important, as the terrorist threat in China worsens, and as China's need grows for reliable partners who can help facilitate its take-off as a global power.

Small: 'Founded on a shared enmity for India, the relationship has in some respects run deeper than formal alliances'

How do both countries profit from this relationship?

Pakistan has certainly been the greatest beneficiary. China has been Pakistan's only wholly reliable arms supplier, its diplomatic protector, and is becoming an investor with a potentially transformative impact on the country's fate. Given how important nuclear weapons are to the South Asian nation's military capabilities and even identity, the significance attached to China's essential role in the nuclear program can hardly be overstated.

But China has derived advantage from the relationship too – Pakistan has been a conduit for Beijing during its years of isolation, and was a crucial intermediary for Beijing in establishing its relationship with the United States, with Saudi Arabia, and with the Taliban. Its intelligence services' ties with militant groups in the region helped, for many years, to ensure that China was never a top-tier jihadi target. When China wants to establish naval facilities in a country it can trust in a crisis, Pakistan is the only reliable choice.

But weighing benefits on a quid-pro-quo basis can be misleading - a strong, capable Pakistan is an asset to China in its own right, given the Pakistani military's continued desire to act as a counterbalance to India. Chinese support to Pakistan acts as an important geopolitical investment, whatever Pakistan gives in return.

It is hard to avoid the fact that China is now a global power, and Pakistan matters far less to China than vice versa - but as China debates shifts away from a non-aligned foreign policy, and upgrades the importance of friends and allies, Pakistan is in a unique position. As one Chinese expert put it: "If China decides to develop formal alliances, Pakistan would be the first place we would turn. It may be the only place we could turn."

How is Beijing supporting Pakistan's nuclear program?

China has been the principal source of external assistance for the Pakistani nuclear program – crucial materials, technical expertise, bomb designs, and even highly-enriched uranium (HEU) were provided by the Chinese side, the only time a nuclear state has given HEU to a non-nuclear state for military use.

China also sold ballistic missiles to Pakistan and helped the country develop its own capacities for missile production. The most important phase of cooperation on the nuclear program was in the 1980s, during the earliest phase of the Pakistani program, and for missile sales in the late 1980s and early 1990s. The initial details of the cooperation program between the two sides appear to have been agreed in the late 1970s and Chinese support has continued ever since, even to the present day.

Chinese backing for Pakistan's program ensured that it could maintain its role as a balancer in South Asia after India developed a bomb of its own, continuing to keep Delhi strategically tied down by its western neighbor. China also hoped to benefit from Pakistan's access to Western nuclear technologies – such as European centrifuge designs – given the deficiencies of its own nuclear program, though in practice this does not appear to have translated into much benefit for the Chinese enrichment program.

The willingness of both sides to cooperate on an area of such extreme sensitivity and crucial importance has provided much of the basis for the unusual level of trust that exists in the relationship – as well as leaving Pakistan in China's debt.

How would you describe China's dealings with the Taliban?

China was initially concerned about the rise of the Taliban, who provided a safe haven for Uighur militant groups after they had taken power in Afghanistan. At Pakistan's instigation, the two sides were able to reach an agreement – following direct meetings by Chinese officials with Mullah Omar – that saw the Taliban promise that their territory wouldn't be used for attacks on Chinese soil, while China moved ahead with commercial cooperation, modest arms sales, and a degree of tacit recognition for the legitimacy of the Taliban government.

That deal has held, and Chinese dealings with the Taliban have continued ever since – some Chinese officials claim that China is the only state other than Pakistan itself to maintain continuous contact with the Afghan Taliban leadership since 9/11. China has sought assurances from the Taliban that it will continue to distance itself from Uighur militant groups, and the Taliban continues to benefit from some money, some arms, and a degree of political recognition from Beijing.

This now puts China in an important position when it comes to brokering reconciliation talks with the Taliban – uniquely, it is trusted by the Pakistanis, the Afghan government, and the Taliban leadership. Beijing doesn't want to see a Taliban takeover of Afghanistan – not least because they doubt the Taliban's capacity to enforce the "deal" – but does want to help facilitate a politically stable outcome there. China's role in this respect was very modest and very discreet but it has become more visible in recent months.

Does China have any plans in the event of a major crisis in Pakistan?

China is certainly anxious about Pakistan's future and the Chinese military's planning for crises there reflects those fears. Some military intelligence officers put Pakistan second only to North Korea in terms of the contingencies in regional states that worry China most. Chinese investments have been constrained by Pakistan's instability and security threats to Chinese workers – by some measures, Pakistan has been the most dangerous place to be an overseas Chinese.

Small: 'The economic relationship has been very weak'

There have also been tensions over Beijing's belief that Pakistan has not been as robust in dealing with Uighur militants operating from its territory as China would have liked – more members of China's terrorism "hit list" were killed by US drones than by the Pakistani army. China has some fears that this reflects rising Islamist sympathies in Pakistan as a whole, including in the army itself.

The economic relationship has also been very weak – many of the most ambitious plans have faltered for reasons that have as much to do with corruption and problems in the Pakistani economy as security threats. The two sides don't even see eye-to-eye even on dealing with India – China would like to see a relationship between Pakistan and India that is less volatile, has a stronger economic component, and more closely resembles the China-India relationship.

What role do China-Pakistan relations play in Asia's geopolitics?

The West has long sought to use China's position of special influence in Pakistan to its advantage. China is one of the only countries with leverage over Pakistan, and during crises in South Asia – such as near-wars between India and Pakistan – Chinese diplomacy has played a crucial restraining role. Outside the context of crises, however, coordination with China has proved far more difficult, and it has still tended to operate as Pakistan's enabler and protector, whether it comes to the use of groups such as Lashkar-e-Taiba or the rapidly growing Pakistani nuclear program.

From an Indian perspective, this is still the greatest concern, and China is becoming less inhibited than it was in the past about certain sensitive areas of strategic support to Pakistan, such as the development of naval facilities or infrastructure projects in disputed Kashmir. In this respect, the role of the China-Pakistan axis in the geopolitics of the region in tying India down in South Asia is still central – India wants to look east, while China wants to ensure that it still has to keep a close eye on its west.

But India, the West and Afghanistan alike have some reason for hope when it comes to the role of the China-Pakistan relationship in Afghanistan's future. China used to outsource its Afghanistan policy to Pakistan but is now taking a far more active role in the planning for the aftermath of the West's drawdown.

While China and Pakistan will remain close partners there, they don't entirely see eye-to-eye – Beijing has a greater interest than Islamabad in Afghanistan's stability and expects Pakistan to take Chinese concerns into account. In the past this might have remained a low priority matter for China but the escalation of terrorist attacks on the Chinese mainland has turned it into a top security concern, and the hope is that Beijing will exert some degree of pressure on Pakistan's calculations.

All sides, including Pakistan, will also benefit if China moves ahead with a major package of investment that could – potentially – help stabilize the Pakistani economy and its energy situation, and turn it into a regional transshipment hub. Chinese investments are not purely for reasons of commerce or strategic economic geography – China knows that its friend is facing an uncertain future and is prepared to make these commitments in part in order to help shore the country up.

Small: 'China was initially concerned about the rise of the Taliban'

How do you expect Sino-Pakistani ties to develop in the coming years?

The relationship remains resilient, and many of the pressures that might have been expected to derail it, such as India's rise, or the growing threat of extremism, have only served to emphasize its continued importance to Chinese interests. Pakistan should be poised to benefit even more from the relationship in future – the growing capabilities of the Chinese military hold out the promise of more advanced equipment, and the vast financial resources that China can now bring to bear may finally transform the weakest element of the two sides' ties.

The relationship hinges on what the Chinese describe as a secular-strategic rationale – with India as its continued focal point – and many of the economic plans still hinge on the security situation in various parts of the country. Pakistan should be one of the biggest winners from China's rise as a global power but if its internal security deteriorates or if Chinese anxieties about the country's political direction worsen, it will be a great lost opportunity.

Andrew Small is a transatlantic fellow with the Asia program of German Marshall Fund of the United States and author of the book "The China-Pakistan Axis - Asia's New Geopolitics." You can follow him on Twitter @ajwsmall.