With the territory of the Islamic State cut by a third and Kurdish militias launching an offensive into its proclaimed capital of Raqqa, the group appears to be preparing its followers for a new, drawn-out phase of warfare.

In a rare recorded audio message released May 21, IS spokesman Abu Mohammed al-Adnani conceded that the group has lost territory to the US-backed international coalition and its allies, and vowed that IS will still strike the West even if it is “driven into the desert.”

He called on IS supporters across the world to carry out attacks during the month of Ramadan, which starts in early June.

“We will make this month, inshallah, a month of calamities for the infidels everywhere,” said Mr. Adnani, also known as Taha Subhi Falaha. “This call specifically goes out to the supporters of the Islamic Caliphate in Europe and America.”

The announcement signals a shift away from the traditional military campaign that enabled IS to rapidly capture large swaths of territory across Syria and Iraq and establish the so-called “Islamic Caliphate,” a jihadist’s utopia where its ultra-extremist interpretation of Islam is enforced in all aspects of life.

IS, which is also known as ISIS, has long based its legitimacy – and supremacy over other jihadist groups such as Al Qaeda – on its ability to capture and administer territories in Iraq and Syria. That was the major draw for an estimated 25,000 recruits who came from Arab Gulf states, European capitals, and the United States.

Analysts and counterterrorism experts say Adnani’s message came as a rhetorical “insurance policy” should it soon be ousted from its strongholds, and suggests that the group is preparing followers to wage a long-term insurgency.

“What the speech is trying to do is set the stage so if they do face that setback, they are able to adapt and keep their followers,” says J.M. Berger, a fellow at the George Washington University’s Program on Extremism, and coauthor of a book on IS. “We are not talking about ISIS going away, we are talking about a major tool of ISIS going away. What we have yet to see is what shape its new approach will take.”

Treasury of $2 billion

In his weekend message, al-Adnani gave an air of invincibility, referring to the ability of IS’s predecessor, Al Qaeda in Iraq, to recover from near devastation in 2008 and take over the northern Iraqi city of Mosul in dramatic fashion six years later.

“Would we be defeated and you be victorious if you were to take Mosul or Sirte or Raqqa, or even take all the cities that were controlled by us and we were to return to our original condition? Certainly not!” Adnani said in the 32-minute speech, published by Al Furqan, IS’s media arm.

The group offered a preview of the new havoc it can wreak even without its strongholds on Monday, unleashing a series of explosions in Tartus and Jableh on Syria’s Mediterranean coast, killing 120.

What made the attacks impressive was not only the scale, but the location – in coastal cities fiercely loyal to the Assad regime, which is fighting IS. Counterterrorism experts are still befuddled about how IS could plant one of its operatives – let alone fully equipped sleeper cells – hundreds of miles away from its territory and in the heart of one of the most heavily-fortified regime havens.

The group in Syria retains 20,000 fighters and a treasury of more than $2 billion, which observers say can fund “several years” of insurgent attacks.

“In Syria, ISIS literally has tons of weapons and explosives, and are likely now to shift their strategy from military campaigns to coordinated car-bombings and suicide bombings,” says Patrick Skinner, counterterrorism expert at Soufan Group intelligence firm and former CIA case officer. “They still have members and weapons and they will shift from harder targets to, sadly, civilian targets anywhere in Syria and Iraq.”

New challenge for US-backed coalition

Unlike other terrorist groups, such as Al Qaeda, IS has been unique in that it did have an "address" – a known headquarters, which Western military forces could target. But should IS choose to go underground, forgoing its geographical base and melding into the local population, experts say the US and its coalition will need to make a dramatic shift in strategy in their two-year war.

Until now, the coalition has relied almost solely on air power, carrying out 9,500 airstrikes in Iraq and Syria since August 2014 to weaken ISIS, decreasing its territory by 35 percent, according to the Pentagon.

In such a traditional military campaign, with air superiority, “it will be a long, expensive fight, but you will win,” says Mr. Skinner. “But when it is a counter-terrorism fight, you need forces on the ground. You can’t win in the air.”

When looking to the next phase of the coalition’s war on ISIS, analysts and military experts point to the campaign against Al Qaeda in Iraq in 2007 and 2008, when government and tribal forces, bolstered by a US military surge, drove the group from its strongholds in western Iraq.

Yet Syria today is far from what Iraq was then. There is no effective local government for US-backed forces to partner with, while a multitude of factions and foreign armies make a unified ground force almost impossible to assemble.

“In Syria, you spend half the time making sure your allies don’t fight each other instead of the enemy,” says a Western military official in Amman, Jordan, who is close to the coalition.

Tensions have already reportedly arisen in northern Syria. Kurdish-led People’s Protection Units (YPG) militias, one of Washington’s main allies in the ground-war against IS, have repeatedly clashed with Arab rebel groups affiliated to the FSA, many of whom are vetted and also backed by the US.

Then there is the question of US commitment. As of April, some 300 US special forces had been dispatched to Syria. If it were to root out IS after the group goes underground, analysts say Washington would have to bring in “thousands.”

The situation in Iraq today is no more conducive to waging a counter-terrorism campaign.

While the US relied heavily on Sunni tribes and their “awakening councils” in 2008, the Iraqi government is now leaning heavily on sectarian Shiite militias, outsiders to Sunni Iraq, who cause resentment among Sunni civilians and reinforce IS’s portrayal of a greater sectarian war.

IS likely to adapt

Analysts say IS will rely on a “hybrid model,” using insurgency tactics such as suicide bombings with a standing militia ready to seize territory should the opportunity arise.

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“The group has adapted throughout its history, and has never truly gone away – always waiting to benefit from the social, political, and economic failures in Iraq and Syria,” says Hassan Hassan, a Syrian fellow at the Washington-based Tahrir Institute for Middle East Policy and coauthor of "ISIS: Inside the Army of Terror." In his assessment, the group could come back stronger if the region doesn’t stabilize.

“Politically and economically Iraq and Syria are only getting worse; new tensions and new divisions are emerging that will, once again, allow ISIS to rise again. It knows this.”