As U.S.-backed forces fight to seize the last bit of territory held by the Islamic State (IS) in eastern Syria, the battle against jihadist influence is far from over. Counterterror experts warn that extremist groups may still try to recruit a rising generation of hundreds of millions of millennials to their ranks.

The U.S.-backed Syrian Democratic Forces on Tuesday announced full control over the remaining IS enclave of Baghuz in eastern Syria after hundreds of IS militants surrendered overnight. The capture was a significant step in the fight against IS, but not a complete victory over the terror group as fighting continued with some jihadists along the Euphrates River.

Some experts said the final push in Baghuz was the end of Islamic State's self-declared caliphate, but IS and other radical Islamist organizations will continue to attract new members because the West has made little progress on the ideological battlefield.

U.S.-backed Syrian Democratic Forces fighters cele U.S.-backed Syrian Democratic Forces fighters celebrate their territorial gains over Islamic State militants in Baghuz, Syria, March 19, 2019. U.S.-backed Syrian Democratic Forces fighters celebrate their territorial gains over Islamic State militants in Baghuz, Syria, March 19, 2019.

"In terms of what comes next, I think these movements adapt very quickly operationally," said Juan Zarate, a senior national security analyst who served as deputy national security adviser for combating terrorism in the George W. Bush administration.

"We will see this with ISIS going underground. We have seen this with al-Qaida adapting and going underground. They will rationalize the loss … in part because they have very long-term visions of their own movements in history. So they will see this as just one chapter, whereas we in Washington who are thinking in two-year cycles, maybe at most in four-year cycles, see this as the end of [IS], or the killing of [Osama] bin Laden as the ending of al-Qaida," Zarate said, speaking Tuesday at the Washington Institute.

Zarate said the defeat will most likely encourage IS to revisit its actions and implement an al-Qaida-style strategy of insurgency while hiding among more vulnerable Muslim communities.

"Part of the ideological clash between al-Qaida and Islamic State was al-Qaida saying, 'Look, we've learned lessons of how to go about doing these terrorist movements. We've learned some very hard lessons that if you pop your head up too much, if you expose yourself too much, you're going to get whacked by the American and the counterterrorism forces aligned with them,' " he said.

Virtual caliphate

Experts say the loss of IS territory or caliphate is likely to prompt the terror group to step up efforts to spread its ideology and recruit followers on the internet. That is because the lost caliphate was an effective tool for inspiring prospective recruits and spreading ideas, and the IS leadership will have to replace that if it is to survive. IS has shown considerable skill in online recruiting, and Western powers have been ineffective in countering IS propaganda, they say.

IS online communication and propaganda over the years has declined as the group lost territory in Iraq and Syria. Nevertheless, the jihadists have continued to recycle old propaganda messages and even create new ones.

IS on Monday released a 44-minute audio recording of its spokesman, Abu Hassan al-Muhajir, calling followers to take revenge for the two attacks targeting mosques in New Zealand that left 50 people dead last Friday.

"The scenes of the massacres in the two mosques should wake up those who were fooled, and should incite the supporters of the caliphate to avenge their religion," he said.

Al-Muhajir mocked the U.S. assertion that IS was defeated, claiming its leader, Abu Bakr al-Baghdadi, was still alive and urging the supporters of the caliphate to retaliate against the U.S.-led campaign in Baghuz.

A U.S.-backed Syrian Democratic Forces fighter pra A U.S.-backed Syrian Democratic Forces fighter prays after returning from the front line in their fight against Islamic State militants in Baghuz, Syria, March 19, 2019. A U.S.-backed Syrian Democratic Forces fighter prays after returning from the front line in their fight against Islamic State militants in Baghuz, Syria, March 19, 2019.

New Zealand attack

Matthew Levitt, a counterterrorism expert at the Washington Institute, said al-Muhajir's audio message after nearly six months of silence shows IS wanted to exploit the New Zealand attacks to incite hate and inflame its anti-Western propaganda.

"They see the opportunity to affect people when they are feeling angry, vulnerable and emotional. And that presence in the virtual world is very, very real," Levitt said during a discussion on The Battle Against Extremism: Assessment and Prescriptions at the Washington Institute.

Levitt said IS most likely would try to restore its image among the vulnerable Muslim communities.

"As we get farther and farther away from what that [IS] caliphate really was in terms of the barbarism, et cetera, they will continue and will have a greater effect at presenting it as, 'Maybe we weren't perfect, but it was a caliphate. Therefore, you need to come and join us again and get back in line to be like the original followers of the Prophet Muhammad,' " he said.

According to Farah Pandith, a former U.S. envoy to Muslim communities, the U.S. and other Western powers need to make sure they step up their efforts to fight back against IS and other extremist groups ideologically.

Pandith said the counterterror strategy after the Sept. 11, 2001, al-Qaida attacks on the U.S. underestimated the importance of battling extremism on the ideological front, leading in part to the emergence of groups like IS.

"We failed in large part because we didn't imagine what could happen. We thought we understood and we had things in a box. We need to reimagine the worst-case scenario ideologically and apply ourselves for that problem, not the problem that we are dealing with today," she said.