Read this CBC News article.
Background: So Pakistan votes for war on its neighbour Afghanistan because Afghanistan is ruled by and supporting terrorists who attack other countries. In addition, Pakistan and Afghanistan have an ongoing dispute over their shared border. Well, to no one's surprise, the vote passes at the U.N.. NATO, with others, invade Afghanistan and successfully force a regime change. Subsequently, the most wanted terrorist, who shared responsibility for the actions leading to this war, lives for years after the invasion, supposedly unnoticed (Likely story!) for quite some time quite close to an important military compound within Pakistan.
According to the article, recently, some Afghan soldiers, near this undisputed border, come under fire from the direction of Pakistan. As one would expect, the Afghan soldiers are not about to put up with being shot at and they call in NATO air strikes. As it turns out, it was Pakistani soldiers or others very close by who were firing on those Afghan soldiers. Pakistani soldiers were killed and two posts were destroyed. As a result, Pakistan has cut of supply routes into Afghanistan, demanded that NATO leave a base in Pakistan within 15 days, and complained bitterly, through diplomatic channels, to Secretary of State Clinton and the Afghanistan foreign minister.
I mean really, what the hell did you expect, Pakistan?