Wikipedia Rip:
The War in Afghanistan began on October 7, 2001,[30] as the US military's Operation Enduring Freedom (OEF) that was launched, along with the British military, in response to the September 11, 2001 attacks on the US. The UK has, since 2002, led its own military operation, Operation Herrick, as part of the same war in Afghanistan. The character of the war evolved from a violent struggle against Al-Qaeda and its Taliban supporters to a complex counterinsurgency effort.
The first phase of the war was the aftermath of the attacks of September 11, 2001, when the United States launched Operation Enduring Freedom, to remove the safe haven to Al-Qaeda and its use of the Afghan territory as a base of operations for terrorist activities. In that first phase, U.S. and coalition forces, working with the Afghan opposition forces of the Northern Alliance, quickly ousted the Taliban regime. During the following Karzai administration, the character of the war shifted to an effort aimed at smothering an insurgency hostile to the US-backed Karzai government, in which the insurgents preferred not to directly confront the International Security Assistance Force (ISAF) troops, but blended into the local population and mainly used improvised explosive devices (IEDs) and suicide bombings.
The stated aim of the invasion was to find Osama bin Laden and other high-ranking Al-Qaeda members to be put on trial, to destroy the organization of Al-Qaeda, and to remove the Taliban regime which supported and gave safe harbor to it. The Bush administration stated that, as policy, it would not distinguish between terrorist organizations and nations or governments that harbored them. The United Nations did not authorize the U.S.-led invasion of Afghanistan.[31]