How did the US respond to the terrorist attack on September 11 2001?
The initial response was a defensive one : all air traffic in the US was suspended for two days while new safeguards were designed. These eventually tightened security for all air travel, domestic and international.
On October 7, 2001, President George W. Bush authorized the military invasion of Afghanistan under Operation Enduring Freedom, which eventually forced the Taliban rulers of the country from power. As they had been protecting the terrorist leader Osama Bin Laden, he was forced into hiding in the mountains on the Pakistan border.
On November 25, 2002, the US established the new Department of Homeland Security to oversee and coordinate all US intelligence and law enforcement in the area of domestic security and terrorism.