On September 11, 2001, four airliners were hijacked by terrorists belonging to the group known as al-Qaeda. Two planes were crashed into the World Trade Center Towers, one was flown into the Pentagon, and another was forced to crash into a Pennsylvania field. The 19 men who hijacked the planes belonged to the terrorist group known as al-Qaeda which was based in Afghanistan under the protection of the Taliban. Osama bin Laden was the leader of al-Qaeda and plotted the attacks on the United States. After the attacks, U.S. President George W. Bush ordered the Taliban to deliver the al-Qaeda leaders to United States authorities. The leader of the Taliban, Mullah Mohammed Omar, refused and the United States began to plan for war.