Recently the government of Pakistan asked the government of the United States of America to stop making remote-guided drone bombing attacks within Pakistan's borders. Friday, April 22, the U.S. defied the Pakistan government and launched an attack that killed 23 people in North Waziristan, including five women and four children. Thus in one stroke President Barack Obama and his administration committed war crimes by becoming (again) an aggressor nation and by targeting civilians.
Ostensibly Pakistan is still a U.S. client nation, but the region is in rapid flux, and likely a covert war between the U.S. and Pakistan has commenced. Neither side will want to declare war at first. The U.S. almost never declares war because it is almost always an aggressor and only Congress can declare war, where one or two Senators are likely to oppose a declaration, and give good reasons. For Pakistan to declare war first is to invite both the destruction of its economy by bombs dropped by the U.S. Air Force, and possibly invasion by India. Neither nation will want to go to the United Nations (U.N.) either. The U.N. was set up to be an instrument of American (and originally, British) imperialism, but the U.S. would certainly get some sound criticism there.
Pakistan was carved out of the Indian subcontinent by the British when they could no longer maintain their empire. By setting up a primarily Islamic state in Pakistan and Hindu state in India, the British maintained a divide-and-conquer strategy. Since India set itself up as neutral but friendly to the old Soviet Union, the U.S. chose Pakistan for a client state. Also troubling was Pakistan's western border with Afghanistan. The border, also established by the British, did not run along tribal lines, instead splitting several large tribes in two (tribes large enough that they would allowed to be nations if they were white Christian tribes in Europe). Throw in Kashmir, with its Islamic majority and Hindu overlords, and the whole area has always been what is euphemistically called geographically unstable.
Both India and Pakistan now have nuclear weapons, but India is a wealthier, more populous, more technologically advanced nation. India has drifted into the U.S. camp during the past 20 years, what with Russia turned capitalist and the vast and rapidly growing Indian-American business community. That has left Pakistan the odd man out, with poor choices all around. China has grown to be more of an ally, but it also is anti-Islamic, and is a dangerous business rival. Afghanistan would make a good ally, but it brings its own problems, including its American puppet government.
Internally Pakistan is in turmoil. In addition to its tribal and class divides, it is in a cultural crisis. It is close to being a modern, technologically (and academically) sophisticated nation, but it also has a majority of people who subscribe to various Islamic fundamentalist sects. Over time it might have undergone more cultural modernization, but the U.S. invasion of Afghanistan (and crimes against Palestine, and Iraq), and the association of the U.S. with Christianity (an even older and more absurd religion than Islam) have done much to revive religious fervor.
Pakistan has a democratic government, and that means you can have factions in power that are pro-Taliban, or pro-U.S., but almost everyone considers themselves a Pakistani nationalist. Given past patterns, the U.S. will now try a variety of tactics. The CIA will undermine, or even attack or assassinate, Pakistani politicians, soldiers, and cultural leaders who are seen as too pro-Taliban (or pro-Chinese). The U.S. will accidentally bomb some Pakistani Army regular soldiers, just to give the generals a taste of what a real war could be like. The U.S. may bribe India to initiate a few skirmishes at the border.
In turn, Pakistan will provide better support for the Taliban. They will let arms and money flow into Afghanistan, and perhaps provide even more advanced weapons. When those weapons hit U.S. troops, the Pakistan military will claim they were stolen. The U.S. will then upgrade bombing runs to include destroying Pakistani army military dumps anywhere near the Afghan border.
After that, everything depends on the political situations in each nation. My guess is that since Barack Obama has entirely alienated the peace voters in the U.S. since taking office, he'll choose to look tough going into the 2012 elections. He can't afford another quagmire in Pakistan, so he'll probably open up unrestricted bombing, just as the U.S. started bombing Cambodia and Laos during the Vietnam War.
As primitive as the Pakistan military is compared to the U.S., it has proximity to its advantage and is several of cuts above the Taliban. The Pakistani Army could be in Kabul in two weeks (after sufficent preparation). Since they would be invited by and welcomed by the Afghani people, they would not be engaging in an act of aggression. Maybe that would be best for everyone, if the U.S. had the wisdom to mind its own business for a change.