Note my point about the taliban in afghanistan- guerillas win wars.
Erm, no. Guerrillas annoy. Guerrillas harass. Guerrillas weaken the foundations upon which a country can be built. They dont win wars. Using your own example, we have won the war in Afghanistan, we just havn't killed them all, or converted them to ourway of thinking. There is no chance that the taliban will win in Afghanistan as they simply dont have the resources or pool of manpower to draw upon.
And to those saying the south is nice and safe, I point to the entire point of the Storm of Chaos, that of the relief forces going to the north. And as far as everyone is concerned, due to the almost total destruction of the northern armies in their desperate rearguard action, they are still there in the majority, as they cannot leave the north undefended (emperor's wishes overall petty count's objections). Therefore, the south is relatively undefended, and if they can get past the border forts, they could have a field day, as most of the knights did return home unlike our southern garrisons.
And if the bretonnians could capture the population centres in the south, it would be victory bretonnia. War is won on production facilities, not land held, and even if the empire held 70% of the arable farmland, and the bretonnian's strikes took the weakly held towns in the south, they would have the MASSIVE producing facilites boasted about as the empire's salvation (which i doubt verily much). And if the bretonnians could get their hands on handguns and cannons... Remember that the southern towns dont have the massive fortifications that the northen ones boasted, and even bretonnians can win sieges (they are not the incompetents that you seem to think they are. Occasionally they do step off their horses

) With southern garrisons runningat an all time low, a besieging army could break into many fragments and take several towns simultaneously, building siege engines and running through most settlements.
Im not saying they could take the empire straight off. They would reach teh reik and be halted by logistics. But they could take most of the south, including our critical town of nuln. However, our culture and values are such that would provoke rebellion, not only in the empire but in bretonnia proper, and so the knights must adapt their feudal values or suffer a revolt, and the land would never truly be fully in their hands for several hundred years, as taking land always breeds recedivists - look at russia pre-communist for examples of autocratic regimes taking other lands (baltics, etc) and russifying them to no avail - hundreds of years later and there was still a freedom movement. Imagine what would happen of the north was still allowed to feed that feeling with propaganda and supplies.
Either way, it would pointlessly drain the resources when they need them most, so it probably wouldnt happen. But the Dark Lord of Moussilon is gaining power and influence, and he hungers for revenge... <insert plot line featuring a nice, shiny new capmpaign featuring zombie knights, zombie pirates and everything zombie relayed - huzzah!>