Justaguy,
There are two ways in which an enemy can be defeated. One is by straight out destruction of his forces on the battlefield. The other is by more subtle means - destruction of his will to fight.
Invariably, any power that has ever attempted to occupy Afghanistan has ended up falling prey to that second one. They can indeed win a series of battlefield victories, but which still don't bring the conflict to a definite conclusion. Their (Afghan) enemies just will never oblige by handing over their rifles and simply giving up the ghost!
This is not at all unique, either, to Afghanistan. When you follow the history of any successful guerrilla type war, you can see the same pattern. One example is the Tet offensive in Vietnam (January-February 1968) that resulted in a heavy battlefield defeat for the Communist forces, yet sufficiently unnerved America and its allies that they lost the will to continue the conflict. Another example is the Algerian War of Independence. During that conflict (1954 - 1962), the FLN suffered a decisive military defeat; yet in the process of handing out that military defeat, the French nation lost its will to keep up the charade in Algeria.
Such "low-intensity" conflicts can never be won by military means alone. That seems to be a lesson the USA, in particular, has failed to learn over and over again.
The British Empire, during its time, achieved some notable victories in Afghanistan. In the Second Afghan War of 1878-1880, Lord Roberts won a brilliant victory at Peiwar Kotal. Later, with a 10,000 man force, he carried out an epic 300 mile forced march to lift the siege of Kandahar - an event which was concluded by yet another decisive battlefield victory. However, by that time, the British at least had the smarts enough not to attempt to colonise the country. Their forces were withdrawn, and from that point onwards, Britain used more subtle means to advance its interests in Afghanistan.