Al Qaeda central is no longer based in Afghanistan, Bin Laden and his core operation are all my accounts now based across the border. They suspected that Pakistan’s Federally Administered Tribal Areas (FATA) is now the origin of al Qaeda, where the central command of al Qaeda located. Therefore they also needed army troops to threaten the terrorists lurked in Pakistan. The Taliban movement in Afghanistan has clear and deep relations with al Qaeda and also compassionate to it, but the evidence is present that shows al Qaeda is also present in Afghanistan today, which could intimidate the U.S homeland in any direct way.
If the American army is removed from Afghanistan then there are chances that the Taliban and al Qaeda revolt becomes strong enough to collapse the Afghan government. And if this government is replaced with a neo-Taliban system of government or the Taliban were able to fortify real political control over some of the major bordering fraction of the Afghan area then maybe al Qaeda re-establish and start threatening the U.S and whole world again. Therefore U.S has to assure the presence of its military enforcement in Afghanistan, Iraq, and Pakistan because they are weak states whose failure could provide havens for al Qaeda and other terrorist groups.
To have a check and balance on Pakistan with the help of political and military pressure is necessary because Pakistan is far richer and better connected to the outside world than Afghanistan and provides an ideal state for al Qaeda in case of its failure. Pakistan is much more perilous because it expected to be a safer place for the al Qaeda, and the risk of nuclear weapons falling into al Qaeda’s hands would be grave indeed. U.S has to stay in Afghanistan and control its military movements, because U.S is now so unpopular in Pakistan, that the U.S cannot deploy major ground forces there to help Pakistan’s government against fighting terrorism and extremism. (Biddle 2009)
The policies of the U.S towards Afghanistan and the Middle East are not changed to such extent as was expected.