If Canada's mission in Afghanistan has shown anything, it is exactly how difficult it is to rebuild failed states.

Mark Sedra, a senior fellow at the Centre for International Governance Innovation and an expert in security and governance in Afghanistan, says Canada is leaving in 2011 not because the job is done but because politicians and the public dramatically underestimated how difficult the task would be.

"We went in there with completely unrealistic expectations, including how long it would take," he said.

"I would say if you don't have the political will to complete the job, don't go in the first place. You can actually make things worse and just wind up wasting your own resources, including the most precious resource of all, your soldiers' lives."

Sedra was one of the speakers at the Kingston Conference on International Security, an annual gathering of academic and military experts in war and reconstruction.

One of the main themes at the conference is dealing with failed states and the problems they present to the west. In the light of Afghanistan, delegates are suggesting future foreign interventions will be smaller and much more limited than a grand plan to rebuild an entire society.

The conference heard that options for failed states range from a Darwinian let-them-fail philosophy to the sort of multi-nation engagement like that in Afghanistan.

Sedra and others at the conference argue that failed states -- Afghanistan is the most pronounced but there a number of others including Somalia, Haiti and the Congo -- can't just be ignored. They destabilize their neighbours, turn to crime including drugs, arms and human smuggling and even become potential pandemic incubators as their health systems break down.

However, it takes decades to rebuild a collapsed state and Sedra fears Afghanistan will collapse again if the international community leaves too soon and could return to being a terrorist staging base if that happens.

Sedra said Canada was not alone in facing domestic political pressure to withdraw. Other NATO nations, not least the United States, are facing the same short of pressure. All were similarly unprepared for how difficult it ultimately proved to be to establish a functioning government and some semblance of a civil society in a country that has had neither for decades.

The conference is examining how the west should best intervene in countries wracked by war and ineffective local governments, when to go in and, perhaps more crucially, when and how to leave.

Sedra said this could be the hardest but most important part. He said the decision to leave is motivated less by the situation on the ground than circumstances back home and always would be in a western democracy.

"Our attention spans are short," Sedra noted.

"If we were serious then we need to say, 'We're not going to set a deadline, we're going to be there for as long as it takes,' but if we walk away and Afghanistan fails again, what will that say about what we've done there? Will people look back on it and see this whole effort as a debacle? I think they might."