Fifteen years after the 9/11 attacks, the shadow of terrorism still lingers. International terrorism is now one of the most pressing challenges faced not only by the US, but also Europe. There is no letup in the efforts to rein in the scourge of terror, however, terrorism is growing and becoming more rampant despite a heavy price paid.

With countries including France and Germany afflicted by a string of terror attacks in recent years, many have accepted that such events are the new normal.

"Terrorists will never be able to defeat the US," US President Barack Obama said during an address to the US citizens on Saturday on the eve of the 15th anniversary of the 9/11 attacks.

But an absurd fact is that Americans have little confidence in the president. According to Pew's latest poll, 40 percent of Americans now believe the US is more vulnerable to terrorism than it was at the time of the 9/11 attacks, hitting a record high.

The US has led the fight against terrorism over the past 15 years. But as Lee Kuan Yew, former Singaporean prime minister, told Fareed Zakaria of Newsweek in 2003, the US made a mistake of "seeking a largely military solution" that will only deal with the tip of the problem.

"In killing terrorists, you will only kill the worker bees. The queen bees are the preachers, who teach a deviant form of Islam in schools and Islamic centers, who capture and twist the minds of the young," Lee warned.

The threat of terrorism has spread and evolved in the past 15 years, as the anti-terrorism endeavor failed to reach the root cause of the evil. The reason why extremism and terrorism are becoming even more pervasive is to a large extent because the benefits of globalization have not been distributed equally and fairly.

In this process, some religious forces were marginalized. They felt insecure and isolated. Problems such as wealth gap and religious conflicts were amplified.

The "new crusade" launched by former US president George W. Bush has worsened the situation, as the war aggravated poverty.

Besides, the development of the Internet and modern communication technologies has facilitated the spread of the radical ideas by the "queen bees," which has led to the emergence of the "lone wolves," an evolving threat that could do greater harm than organized terrorism.

Fifteen years after the 9/11 tragedy, the world is facing a tougher challenge. Tackling the thorny problem requires reflection upon the root cause of terrorism and failures in the past anti-terror wars.