9/11 20 years on: What US 'anti-terrorism' wars left behind

Psychological shadow of terrorism remains

According to the US Department of Homeland Security, as the 20th anniversary of the 9/11 terror attacks approaches, there have been concerns of triggering extremist attacks.

The specter of the attacks in 2001 lingers, serving as a constant reminder. In Washington, for example, the unofficial slogan of the post-9/11 US-"If you see something, say something"-can still be seen on billboards and public transportation. At airports, tightened security measures have made travel more stressful than ever for passengers and reduced their privacy.

About 50 percent of US citizens said they are "extremely concerned" or "very concerned" about the threat to the country posed by extremist groups based outside the US, according to a poll by The Associated Press-NORC Center for Public Affairs Research.

However, about two-thirds of respondents said they are "extremely concerned" or "very concerned" about the threat of extremist groups based in the US.

People's worries are not unreasonable. There have been terror attacks since 9/11 such as the London terrorist attack on July 7, 2005, leading to 52 deaths and over 700 injuries; the Mumbai bomb attack on July 11, 2006, leading to 209 deaths and over 800 injuries; 2011 Norway attacks leading to 77 deaths; the Nov 13 Paris terrorist attack in 2015 leading to 137 deaths and over 300 injuries; and the New Zealand mosque shooting incident on March 15, 2019, leading to 51 deaths and 50 injuries, etc.