UCDP Conflict Encyclopedia:

On US territory, al-Qaida launched its first devastating one-sided attack on the World Trade Center in New York on 11 September 2001, simultaneously attacking government targets in Washington D.C.. These assaults marked the beginning of an intrastate conflict against al-Qaida, which was, however, mostly been fought outside the USA. In 2003 and 2004, al-Qaida carried out a number of attacks in Saudi Arabia which targeted foreigners living there, mainly Americans.

Source: Uppsala Conflict Data Program (Date of retrieval: 2014-08-13) UCDP Conflict Encyclopedia, Uppsala University

Wikipedia

The War on terrorism or War on terror (abbreviated in policy circles as GWOT for global war on terror) is a global effort by the governments of several countries (primarily the United States and its principal allies) to destroy international groups it deems as "terrorist" (primarily radical Islamist terrorist groups, including al-Qaeda) and ensure "rogue nations" no longer support terrorist activities. It has been adopted as a consequence of the September 11, 2001 attacks on the United States.

The United States, with support from the United Kingdom, Australia, Canada and the Northern Alliance, invaded Afghanistan in October 2001 as part of its "War on Terrorism" campaign. The military campaign, led by U.S. general Tommy Franks, was initially dubbed Operation Infinite Justice but quickly renamed Operation Enduring Freedom, due to perceived religious connotations of the former. British military operations against Afghanistan were codenamed Operation Veritas.

According to the U.S., the purpose of Operation Enduring Freedom was to target Osama bin Laden, suspected of planning and funding the September 11, 2001 Terrorist Attack, and his terrorist network al-Qaida, as well as the Taliban government in Afghanistan which allegedly provided support to al-Qaida and gave them safe haven.

Source: Wikipedia, published under the GNU FDL. Retrieved [dat]