On September 11, 2001, four planes were hijacked. The hijackers then deliberately flew three of the planes into two important buildings, the Pentagon in Washington DC and the Twin Towers of the World Trade Center in New York. The fourth crashed into a field near Shanksville, Pennsylvania. The loss of life and damage that these hijackings caused form the biggest act of terrorism ever on United States soil. Nearly 3000 people died in the attacks and the economic impact was immense.
The attacks have greatly increased attention to national security in the United States. This has had huge implications for United States national and international politics. This is particularly true for the relationships between the United States and Islamic countries in the Middle East.
(Taken from: http://www.timeanddate.com/holidays/us/patriot-day)