It is highly unlikely that NATA/US forces can win the war in Afghanistan unless someone cleans out the Taliban and al-Qaeda militants along the border between Pakistan and Afghanistan. And its unlikely that Pakistan's military has the wherewithal or the inclination to do this on its own.
At the same time, America is determined to win the war they launched in Afghanistan in response to the 9/11 attacks. The thought of losing this particularly symbolic war is unthinkable. A way must therefore be found for Pakistan to accept direct American or NATO involvement within its borders. If not, the Americans, probably without the rest of NATO, will continue incursions in force into what has become the Taliban and al-Qaeda terrorist safe-zone.
And should America continue their incursions without at least Pakistan's tacit approval, the new government might fall, opening the way for a takeover of that country by Islamic extremists. And by the way, Pakistan has nuclear weapons.