Talibans cross the border from Pakistan where they are "hosted", commit their crimes and return to Pakistan, without being caught. The place, its orography, really makes that very easy. But there are other factors, notably the lack of action against the Taliban from the "Pakistani Frontier Corps", denied by Pakistan of course:

While the governments involved sort out who fired at whom, it's worth noting that U.S. troops fighting along the border have long contended that the Frontier Corps - an almost exclusively Pashtun tribal militia overseen by Islamabad - has been viewed as often aiding or abetting Islamic insurgents. Pakistan defends the force by saying they have suffered hundreds of casualties fighting extremists.

Afghan President Karzai has menaced to send the Army to the frontier and to enter Pakistan, with strict orders to capture the Taliban leaders there, including Mullah Omar. The statement has produced a diplomatic problem between the two countries. The fact that Pakistan has nukes is something which must be considered before any kind of action is taken.

And the fact that Pakistan has made several deals with the Taliban since the new government took place is also another thing to consider. Among other deals, Pakistan has freed one of the most important Taliban leaders: Sufi Mohammed, the leader of Tehrik Nifaz-e-Shariah Mohammadi (Movement for the Implementation of Mohammad's Sharia Law).

But what comes ahead can be told by two pieces of news: the Taliban fighters have begun to inflitrate in the city of Kandahar and the citizens have begun to flee from the city.