The current crisis between Iran and the Taliban became inevitable when Mazar-i-Sharif, the main base of the Iranian-backed Afghan opposition, fell to the Taliban in early August. Until then, Iran still had a toehold in northern Afghanistan and there was a chance that the opposition might manage a power-sharing deal with the Taliban.
This is unlikely now that the Taliban controls 90 per cent of the country. This means Iran will have to try to keep the regime off-balance by promoting guerrilla warfare by the opposition. Iranian-Taliban hostilities are just another episode in the "Great Game" which has been played by great and small powers in this region for more than a century.
Originally, the clash was between Russia and Britain over India.
Today, Iran stands alone against Pakistan, Saudi Arabia and the US in the competition for Central Asian oil routes.
But the game being played out between Iran and the Taliban is about more than power politics and petrol.
Tehran and Kabul confront each another along a cultural fault-line which divides west Asia from central Asia.
Afghanistan is a rough and ready tribal society, while Iran is the product of a long-established, highly-sophisticated urban civilisation.
The Afghan and Iranian peoples represent opposing camps in Islam: the majority orthodox Sunnis and the 10 per cent minority Shias which broke away in the eighth century and have never really reconciled with the Sunnis.
This ancient schism has been transformed into military confrontation because the Afghans, under the fanatical, tribalist Taliban, seek to carry the "banner of Islam" to the worldwide Muslim community and convince other Muslims to adopt their religious and social practices.
When Ayatollah Khomeini proclaimed his Islamic Republic in 1979, Iran seized the "banner of Islam" and attempted to export its Shia "Islamic revolution".
This is an endeavour which it has now largely abandoned, without it ever having achieved much success. But this does not mean that Iran is now prepared to hand over the "banner" to the Taliban. The conservative faction takes pride in the theocracy installed by the Ayatollah, where behaviour is regulated by Muslim law, women envelop themselves in chadors and external influences are strictly excluded.
Although the Taliban has followed a similar route with the object of establishing an "Islamic state", the conservatives in Tehran contend that the Afghans are so extreme as to be "un-Islamic" or even "anti-Islamic".
The reformists, led by Mr Muhammad Khatami, elected to the presidency in May 1997, just as strongly oppose the Taliban as they condemn oppression at home.
Mr Khatami has begun to transform the backward-looking, authoritarian, repressive Islamic Republic into a democratic, progressive, liberal country where the clerical caste no longer rules and there is freedom of speech and association and the rule of law.
Although his reforms are being fought tooth and nail by the conservatives, he has been able to effect considerable change because he has more than 70 per cent of the people behind him. This is a major development in the Muslim world, where most governments are authoritarian.
If Mr Khatami succeeds, he will create a truly revolutionary trend, which cannot but attract Muslims, both Sunnis and Shias, in all parts of the Islamic world. If he fails, the Taliban could capture the "banner" to the detriment of Islam and the global Muslim community.