Jihadist groups based in the country, some with transnational agendas like al-Qaeda, now have a template for defeating governments backed by major powers and have been emboldened by the Taliban's lightning-fast advance. This is happening as the jihadi ecosystem is experiencing the lowest counter-terrorism pressure in the last two decades, effectively getting free rein.

Asfandyar Mir, south Asia security analyst for the US Institute for Peace, says it's a dangerous combination when threats go up at the same time efforts to combat them go down.

"Central-Asian jihadists have been flexing their muscle, anti-China jihadists have attacked Chinese personnel in Pakistan, more regional violence is extremely plausible - the threat is ongoing, and we are just talking about an escalation from this point onwards," Mir says.

The collapse of the Afghan republic following the US departure would have regional significance like the post-9/11 invasion, or the withdrawal of Soviet troops and fall of the communist regime they'd backed, he says.

"This is a seismic shift that will change politics in this part of the world in [many] ways."

Expect the immediate danger to be regional - in south and central Asia - as geography and capability limit the initial damage. Chinese interests in Pakistan have already taken a hit.

In April, a car bomb exploded at a luxury hotel hosting Beijing's ambassador in Quetta, not far from Taliban strongholds in southern Afghanistan. The attack was claimed by the Tehrik-i-Taliban Pakistan, or the Pakistani Taliban, a loosely organised terrorist group with ties to al-Qaeda, based along the vast Afghan-Pakistan border.

Last month, a bomb blast on a bus travelling to a dam and hydro-electric project in Dasu, near the Pakistan border with China, killed 12 people, including nine Chinese citizens. No one has claimed responsibility, but Beijing was so concerned that it hosted Taliban representatives for a meeting with Foreign Minister Wang Yi.

At stake is $US60 billion in projects in the China-Pakistan Economic Corridor, a crucial part of President Xi Jinping's wider Belt and Road Initiative, along with significant Chinese mining interests inside Afghanistan.

While this wasn't the Taliban's first visit to China, the seniority of the Chinese representatives was unprecedented, as was the very public message that Beijing recognises the group as a legitimate political force, accoring to Yun Sun, the Stimson Center think tank's China program director, in an essay on the national security platform War on the Rocks.

After posing for photographs with the group's co-founder and deputy leader Mullah Abdul Ghani Baradar, Wang described the Taliban as "a crucial military and political force in Afghanistan that is expected to play an important role in the peace, reconciliation, and reconstruction process of the country".

What Beijing wants in return is for the Taliban to live up to a commitment to sever all ties with terrorist organisations, including the TTP and the East Turkestan Islamic Movement (an outfit Beijing blames for unrest in its Xinjiang region, that Washington removed from its list of terror groups in October after finding there was no credible evidence it continues to exist).

Any further attacks on Chinese nationals working in south Asia, whether claimed by the Taliban or others operating with its blessing, will no doubt impact future ties, though it's unclear what China would do in retaliation.

With no major political or diplomatic push to blunt the Taliban's advance or rein in the groups operating in its shadow, including al-Qaeda - much diminished 20 years after the US invaded Afghanistan to destroy them and their Taliban hosts - it's a matter of when, not if, there's an upsurge in terror attacks.

The danger is particularly acute for the six countries bordering Afghanistan. Beyond China, they include Iran and Pakistan as well as nearby India, which will be closely watching its only Muslim-majority province of Kashmir, the object in two of its wars with Pakistan, for resurgent violence. Russia will be concerned about the impact on Uzbekistan, Turkmenistan and Tajikistan, and any terrorist blowback onto its territory.

There's the possibility that the major powers - the US, Russia and China - might step in and convince their allies and friends to end hostilities. But analysts think that's unlikely. The situation has festered since the US and the Taliban reached their agreement in February last year, and will continue to do so.

Extended international inertia is more probable. Look at Syria. After a decade of war and some significant US investment - in the form of money, military involvement and political capital - Bashar al-Assad is still president.

The country has the world's largest population of internally displaced people (6.7 million), while 6.6 million refugees subsist mostly in Lebanon, Turkey and Jordan. The threat posed by terror groups operating in and around Syria, as well as the use and proliferation of chemical weapons, remains a real concern. So does the conflict's tendency to be a flashpoint for external players like Russia, Turkey, Israel and Iran.

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For Afghanistan, the next worry would be that foreign fighters again start pouring in from around the world. Insurgents from other nations are there now, but mostly from neighbouring countries. Once they come from further afield, it increases the probability of attacks spreading much more widely.

Husain Haqqani, Pakistan's former ambassador to Washington and now director for south and central Asia at the Hudson Institute, says the Taliban remain connected to al-Qaeda and other international terror groups by ideology, shared finances and training, and even marriage.

"Given that jihadists do not think much of international borders and consider the current global order un-Islamic, it is only a matter of time before they set their sights on [the West] again," he says.

It's hard to see how this ends well. Unless major powers do more than hold their collective breath and hope for the best, the fallout from their indifference will be felt well beyond Afghanistan's borders.