It emerged this week that Afghanistan's president, Hamid Karzai, has been accepting "bags of money" from Iran, reportedly as transparent aid to help cover palace expenses. Although Iranian money and weapons for use against Nato forces have been pouring for many years into Herat, in western Afghanistan, the nonchalance of President Karzai's response to Iranian cash in Kabul raised eyebrows in the media.

Perhaps they weren't looking far enough afield. The worry is that Iran has similar designs on Pakistan and that the methods will be the same – drip money into the Pakistani Taliban in a long-term effort to destabilise the state. Even if the tactic doesn't work – and there are strong arguments that it will not – it may make things more complicated.

For one thing, it is not known the extent to which Iranian money is swishing around in Pakistan, though much of it will be linked to drugs. From an Iranian point of view, too, their neighbour to the east may look vulnerable.

India's bulwarks

Sectarian and political violence has reared up over the summer in Karachi – both the starting point of Isaf supply lines and the region's major drugs port. Pakistan is also dealing with the aftermath of catastrophic flooding; much of the agrarian economy is destroyed for this year. And the government is constantly portrayed as weak, even if things may not be quite as bad as they seem.

The fragility of Pakistan comes with caveats, something that the Iranians may not have observed. AP reported in September that there had been 3,600 deaths from extremist Taliban attacks in Pakistan since 2007 – the majority of them chronicled in the western press – a relatively low figure, given Pakistan's population of 175 million. In Karachi this year alone, the city's sectarian and political violence has claimed 1,100 lives.

However, the country is resilient. David Pilling wrote perceptively last week in the Financial Times (registration required) that it's a country that refuses to fail. It is unlikely that Iran will be able to further their ambitions for aggrandisement this year or next. But, as with China, which has invested $248m in Balochistan's Gwadar port to protect its oil and gas supply routes in central Asia, the Iranians are in situ.

The concern is that, over the next decade, they will continue to stay there and that the western and eastern regions of the subcontinent – India's bulwarks – will find themselves under pressure.

Reversing the hostility of decades, India began negotiating with Bangladesh in 2009 to wrest the country out of China's insistent clutches. Delhi cannot budge on Kashmir, of which Arundhati Roy spoke bitterly this week, not because it fears Pakistan, but because it has long-term territorial concerns about China.

The building of an empire

Kashmir is India's land corridor to the borderlands of the Himalayas and there are four disputed areas, three of which border Tibet. The fourth, Arunachal Pradesh in the north-east, next door to Bhutan, is claimed by China. India's state security headache prima inter alia is the Maoist/Naxalite rebellion in the north-east, which quite possibly receives funding from Beijing.

Historically, the Persians and the Mongol Chinese have been at it before, because the subcontinent is fertile, and has abundant land as well as the advantage of prosperous seaports. The ancient tribal and Dravidian people of the subcontinent had to put up with wave after wave of Arya peoples from central Asia over 3,000 years ago. Colin Thubron in the Lost Heart of Asia states – wrongly, I think – that five million died in India as a result of Tamerlane's incursions from 1398.

At the start of the 16th century, Babur, the quasi-Persian prince from Transoxiana, began his progress into the rich and properous lands of northern India which heralded the Moghul empire.

The subcontinent has a habit in the past of splintering under outside pressure. However, this time, in the 21st century, if Pakistan, India and Bangladesh can recognise the external threats, they might see that their future is bound up with each other.

Catriona Luke is a freelance writer and editor.