The fall last week of Raqqa, Syria, capital of the self-declared caliphate of the Islamic State extremist group, is a reason to rejoice. The mass surrender of insurgents who had vowed to fight to the death is an indicator of the state of an organization that had an impressive string of victories in Iraq and Syria. At its apogee, IS controlled land equivalent in size to the United Kingdom and reigned over 10 million people. A concerted push by allied forces has pushed IS out of all but a few remaining outposts and forced its members to flee.

This victory does not mean that the war is over, however. IS has lost a caliphate and its leaders driven underground, but it has vowed to return to its roots as a guerilla organization and take up terrorist activities with new determination. Vigilance and equal determination from law enforcement and security agencies are needed now more than ever.

IS leaders and adherents are well acquainted with the vicissitudes of war. Formed in 1999 as an Islamic militant group, it later emerged as al-Qaida in Iraq and played an important role in the chaos after the United States-led invasion of that country. The military surge in 2007 and 2008 inflicted heavy losses on the group — it was estimated that the Islamic State of Iraq, the IS predecessor, had just 700 fighters by the time U.S. forces withdrew from Iraq in 2011 — but it tapped resentments of the Sunni community after a Shiite government took power in Baghdad to sustain itself.

The incompetence and intolerance of the Iraqi government fanned the flames of Sunni grievance, which led to a resurgence for IS. The bloody civil war in Syria provided a second front. IS fighters won a string of victories that culminated in the June 2014 declaration of a caliphate that would rule the world in the name of Islam.

Those successes won followers hailing from countries around the world. They also focused the attention and efforts of governments seeking stability in the Middle East or which felt threatened by a militant Sunni presence. A counteroffensive was launched at the end of 2015 that has unraveled the caliphate and restored most of the land seized to the governments in Baghdad and Damascus.

The war will continue, however. IS remains a formidable force, with an estimated 6,000 to 10,000 fighters in Iraq and Syria, eight to 14 times the number it had in 2011. More ominously, remaining forces are true believers.

The chief concern now is that fighters will disperse to other parts of the world, taking up residence in or creating new IS cells. IS reportedly began planning for defeat over a year ago and is ready to resume as an insurgency. Counterterrorism officials warn that sleeper cells have been established in the Middle East, Europe and Asia, and fighters trained for battle in Iraq and Syria will instead use their skills there. After all, even as IS was losing ground in its caliphate, it claimed responsibility for terrorist attacks in Britain, Turkey and seven other countries — all in 2017 alone.

Asian governments must be alert to the militant presence in Southeast Asia. The Maute group in the Philippines declared itself the Islamic State of Lanao and pledged allegiance to IS in 2015. Philippine President Rodrigo Duterte declared victory over Maute on Oct. 17, after retaking the city of Marawi from the group, a battle that took five months and over 1,000 lives. Many fighters left the caliphate to join that struggle. Other disaffected Muslim groups in Southeast Asia could also be ripe for recruitment.

One of the most worrisome elements of IS is its ability to inspire followers to take action on their own. IS has proved masterful in its use of social media, capable of recruiting the angry and the disaffected all over the world. Adherents do not need formal training and IS directed some attacks remotely.

There are several obvious lessons. First, security officials must anticipate stepped-up attacks after IS defeat. Particular attention needs to be paid to its social media and online presence and attempts to recruit and direct followers.

Second, the IS defeat creates opportunities for rival militant groups to reclaim status and prominence in the fight against the West. Al-Qaida has been chaffing since being eclipsed by IS. It has promoted Hamza bin Laden, the son of Osama, as its leader to attract young followers.

Third, governments must remain sensitive to Sunni grievances. Islamic State, al-Qaida and other militant groups have followers because they are angry, disenfranchised and marginalized. IS spokesman Abu Muhammed al-Adnani, killed last year by a U.S. drone strike, told followers that “True defeat is the loss of willpower and desire to fight.” He is right: The best way to stop terrorism is to give the terrorists no reason to take up arms.

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