According to some reports the Chairman of the Joint Chiefs warned Pakistan the U.S. would act unilaterally to stop Taliban incursions if Pakistan doesn't act and army sources have been cited saying the border forces were put on red alert to avert cross-border incursions. This too follows a major coup for the Taliban insurgency in Afghanistan after they assaulted a U.S. outpost, killing nine U.S. soldiers and ultimately forcing the withdrawal of U.S. forces from the area, which immediately fell under Taliban control. It also follows up the recent bombing of the Indian embassy which Afghanistan accused of being carried out by Pakistani intelligence, as well as an assassination attempt on President Hamid Karzai. Afghanistan halted meetings with Pakistan in a response to the allegations.
NATO forces in Afghanistan have also been launching cross-border strikes following attacks from inside Pakistan. Some reports are indicating the Taliban are intending to spark a cross-border incident, possibly hoping to start a war between the two countries. Drawing Afghanistan and the U.S. into the tribal areas would allow the Taliban to inflict heavy casualties on the forces as well as distract them and Pakistan from its own activities in both countries. While the remaining Taliban forces around Kandahar are believed to have been expelled by U.S. forces the Taliban still controls a substantial portion of the province and could launch a major offensive against the city once more, especially if NATO and Afghan troops are tied down in a major conflict within Pakistan or against Pakistan.
Kandahar is crucial in Afghanistan as a major roadway. Controlling Kandahar would put the Taliban in a prime position to launch an attack on Kabul. Given their already substantial positions around Kabul, which is closer to the main Taliban sanctuary in the Federally Administered Tribal Areas of Pakistan, the seizure of Kandhar could pave the way for a major offensive on the capital. Helping this effort even further would the seizure of another important city in Pakistan, Peshawar. Peshawar is a crucial roadway into Afghanistan and is used to get many of NATO's supplies, it also has been put in serious danger by Taliban positions near the city. Some of these militants were repelled but the main Taliban forces remain and reportedly could take the city at any time which would not only be a boon to the Taliban insurgency in Afghanistan could pave the way for a future assault on Islamabad.
The main source of contention has been the newly-elected Pakistani government's tendency to sign ceasefires with the Taliban only to have the Taliban build up its forces for more major assaults in Pakistan and beefing up the insurgency in Afghanistan. Each time the Taliban comes back stronger and more capable than ever.
If the Taliban is seeking to spark a conflict between Afghanistan and Pakistan it would have no better opportunity than when the U.S. and Afghan forces are straddling the Pakistani border. However the consequences of this could be far broader in scope. A U.S. incursion, if intercepted by Pakistani forces, is likely to spark a border war with the country and India may also get involved in the situation with some suggesting Indian troops be deployed in Afghanistan. Indian involvement in Afghanistan would build up pressure on Pakistan and could increase the risk of an Afghan-Pakistani conflict starting a war between India and Pakistan.
Whether India gets involved or not it is unlikely China will not react to a conflict between the U.S. and Pakistan. However, they are more likely to threaten intervention and rattle sabres at India to prevent their intervention. Aside from building up troops on the Pakistani and Afghan border, naval movements in the East China Sea could also be used to increase the pressure.The threat of war with China and the danger of the Taliban using instability to seize both Pakistan and Afghanistan would likely be enough to bring an end to any conflict between the two parties.
While any U.S. buildup on the Pakistani border is likely just pressure on Pakistan's government to act against the Taliban the potential for a single incident to spark a major confrontation is high and the consequences of a broad and dangerous scope.