President Donald Trump's pronouncement that the Islamic State has been vanquished in Syria and his controversial decision to withdraw U.S. troops from the war-torn nation have raised the question of whether the beleaguered militant group really has been beaten.

"We have defeated ISIS in Syria, my only reason for being there during the Trump Presidency," Trump tweeted Wednesday before ordering the withdrawal of more than 2,000 troops.

A day later Trump acknowledged that more fighting might need to be done, but that the U.S should leave it to Iran, Russia and Syria.

"Does the USA want to be the Policeman of the Middle East, getting NOTHING but spending precious lives and trillions of dollars protecting others who, in almost all cases, do not appreciate what we are doing?" Trump tweeted. "Do we want to be there forever?"

Most experts agree that the military might of ISIS has been severely diminished. But Trump's decision fell flat in his own administration, with Defense Secretary James Mattis resigning the next day. Then Brett McGurk, the U.S. envoy to the coalition fighting the Islamic State, announced his resignation.

Leaders of his own party, including Sens. Marco Rubio of Florida and Lindsey Graham of South Carolina, quickly registered protests. Graham added that Trump’s action would represent a “big win for ISIS, Iran, Bashar al Assad of Syria, and Russia.”

Not everyone, however, disagrees with Trump.

Harry Kazianis, director of defense studies at the Center for the National Interest, says the U.S. had destroyed ISIS. He says he doubts that "we may never be able to kill the last man," whether we have soldiers in Syria or not.

"The challenge with taking on a group such as ISIS is it’s like taking a baseball bat to a beehive," Kazianis told USA TODAY. "You can crush the hive – in this case ISIS and its so-called caliphate. You can kill all the bees – think their soldiers and military equipment. However, there will always be a few bees left that can sting you – that means terror attacks, cyber operations and perhaps even attacks on the U.S. homeland."

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Kazianis says smart intelligence, a clandestine CIA footprint, cyber-snooping and other intelligence methods can ensure that "ISIS never rises again." Iran and Russia have more influence than the U.S. in Syria, he says, and 2,000 U.S. troops won't change that.

Penn State professor James Piazza recently completed work on a study showing that as ISIS has lost military ground, it has increasingly resorted to terror attacks outside its proposed caliphate in Syria and Iraq.

"Most people suspect ISIS is transforming into a more traditional terrorist insurgent group now that it has lost its caliphate," Piazza said. "It still has fighters in Iraq and Syria, but they have gone underground."

Piazza forecasts that ISIS will continue to commit attacks while it awaits an opportunity to re-emerge as a military force. He says Trump's decision to remove troops from Syria is a risky one.

"I think there is a legitimate fear that with President Trump’s withdrawal of troops, ISIS could have an opportunity to revive," Mattis said.

Piazza says the weakness of the Iraqi government and Syrian unrest related to the Arab Spring encouraged ISIS to attempt to build its caliphate. He added that removing troops could expose the Kurds to brutal attacks. The Kurdish militias were critical to U.S. efforts to fight ISIS in both Iraq and Syria.

"If removing U.S. troops introduces more instability into the region, ISIS could exploit that and reconstitute itself," Piazza said. "It begs the question of what our policy toward ISIS currently is. Are we just declaring victory and going home, assuming that ISIS is beaten once and forever?"

British Defense Minister Tobias Ellwood said he believes ISIS is far from conquered.

"It has morphed into other forms of extremism and the threat is very much alive," he said.

French Defense Minister Florence Parly tweeted that ISIS "is not scratched from the map, nor have its roots. It is necessary to defeat militarily the last pockets of this terrorist organization."