The document summarizes the September 11, 2001 terrorist attacks and discusses terrorism as a form of "invisible warfare". It describes how al-Qaeda hijacked four planes on 9/11, crashing two into the World Trade Center towers and one into the Pentagon. Many firefighters sacrificed their lives trying to save people trapped in the burning buildings. In response to the attacks, the U.S. tightened security through laws like the Patriot Act and formed the Department of Homeland Security. The document characterizes terrorism as an unpredictable "invisible warfare" since victims have no warning of attacks, unlike conflicts between nations.