As my old man has said from day one
'you can't fight a war against guerrillas '
Not entirely true - we have won a few guerrilla wars - The Boer War and The Malayan Emergency come to mind off the top of my head.
The only way to win a guerrilla war is to cut the guerrillas off from the people who they need to support them. We did this in South Africa/Malaya by rounding up the local populace and putting them into guarded camps and controlled the food supply. The guerrillas were cut off and starved into submission and gave in.
Only problem is that that sort of tactic is no longer politically acceptable - especially since in the boer war, the camps were called "concentration camps". Another great British Invention there...
As for the Taleban - they can not win the war. But they can't lose either.
All the Taleban have to do is go home and wait for the Western Forces to leave and then start again. The Afghan Establishment is completely corrupt and riddled with tribal sectarianism. The concept of the Nation State does not exist outside of Kabul - the rest of Afghan society owes it's only alliegence to their own tribe. Just as happened when the Russians withdrew, when the foreign invaders have gone the Afghans will end up in another civil war for tribal dominence.
Sadly, we broke the Montgommery's second rule of Warfare (Never Invade Afghanistan). You though we'd learn from the previous three attempts....