September 11 attacks
After the attack, the United States blamed Al-Qaeda, which the U.S. thought was a terrorist group. President George W. Bush said he would start a "War on Terror". He meant that the United States would do more things to try to stop terrorism in the future. Bush said this was meant to protect Americans and their property from terrorists. For example, the American government would be reorganized. Security and control in public places was made stronger, especially at airports. Americans were told every day whether there was a serious threat of terrorism. (This was done by giving a color for the day. Red meant there was a high risk, green meant a low risk, and there were many levels in between.)