The tragedy of September 11, 2001 had enormous impacts on both military and civil aviation. Since then, airport security has been far stricter, while the US military air defense has also changed. A significantly greater percentage of Air National Guardsmen became combat veterans after 9/11 compared to their pre-9/11 counterparts. 9/11 ushered in a two-decade-long era for the United States Air Force and Air National Guard.