Pakistani journalist Ahmed Rashid, who literally wrote the book on the Taliban, offers an analysis of how the U.S. and NATO forces have fallen short in Afghanistan.

An excerpt from the BBC commentary: 

Here is a checklist of failures in the south that the US, Nato, the UN and the Afghan government should be discussing and rectifying:

Washington's refusal to take state building in Afghanistan seriously after 2001 and instead waging a fruitless war in Iraq, created a major international distraction which the Taleban took advantage of to slowly rebuild their forces.

US-led coalition forces were never deployed in southern Afghanistan in sufficient numbers, even though this was the Taleban heartland and needed to be secured. Apart from a US base for 3,000 troops in Kandahar and a couple of fire bases, for four years there was virtually no military presence in three of the four provinces. US forces failed to secure even the major cities and highways in the south. The growing security vacuum in the south was steadily filled by the Taleban.

The rise in attacks come as Nato boosts its troops in the country

Afghanistan has received far less funds for reconstruction than almost all recent nation building efforts such as the former Yugoslavia, Haiti or East Timor. The lack of security in the south meant that UN development agencies and western and Afghan aid organisations could not provide sufficient aid and reconstruction. Nor was there ever adequate funding by western donors, especially for rebuilding the vital agricultural sector. The West's refusal to invest in agriculture on which 70% of the population depend, led to a massive return to poppy production by destitute farmers in the south, which quickly spread to the rest of the country.

Drug smugglers and cartels now offer much greater incentives to Pashtun farmers than aid agencies. The best functioning extension programmes for farmers are operated by opium traffickers who provide improved varieties of poppy seeds, fertilizer, improved methods of cultivation, banking and loan facilities and organised large scale employment during the poppy harvest. Compared to 2001 when poppy cultivation was at a minimum, southern Afghanistan now needs to develop an entire alternative economy costing billions of dollars in order to replace the drugs economy.

The drugs economy has fuelled massive corruption among government officials, undermined the authority of the government and funded the Taleban. The failure to reconstruct the south has led to widespread public disillusionment, increasing sympathy for the Taleban and anger at the Afghan government. Drugs money has allowed the Taleban to acquire new weapons, provide salaries to fighters and larger sums to suicide bombers.

He also talks about the botched attempts at reconciliation with and rehabilitation of Taliban who wished to lay down their arms, and the sanctuary that the Taliban have found in western parts of Pakistan, notably the Northwest Frontier Province and Baluchistan.

Here's an important related post.