The critical event in 2001 that led to increased security measures in the U.S. and globally was the September 11 attacks. Coordinated by the terrorist group al-Qaeda, the attacks involved the hijacking of four commercial airliners, resulting in the destruction of the World Trade Center and damage to the Pentagon. This devastating event prompted the U.S. to initiate the War on Terror, leading to increased security protocols in airports, surveillance measures, and the establishment of the Department of Homeland Security. The attacks fundamentally shifted global security policies and international relations.