Washington: Hours before President Barack Obama announced a new US military offensive against the Islamic State of Iraq and the Levant (Isil), one of his top counterterrorism officials testified to Congress that the Al Qaida offshoot had an estimated 10,000 fighters.

The next day a new assessment arrived from the CIA: The terrorist organisation’s ranks had more than doubled in recent months, surging to somewhere between 20,000 and 31,500 fighters across Iraq and Syria.

The enormous discrepancy reflects, in part, significant uncertainty among US intelligence agencies over the dimensions and danger posed by America’s latest Islamist adversary.

But the trajectory of those numbers — and the anxiety that they have induced among US counterterrorism and military officials — also helps to explain Obama’s decision to go to war against an Islamist group that has yet to be linked to any plot against the United States.

In his speech, Obama laid out a rationale that leaned heavily on what-ifs. The United States has “not yet detected specific plotting against our homeland,” Obama said. But Isil leaders “have threatened America and our allies,” he said, and are on a path to deliver on those threats “if left unchecked.”

The emphasis on hypotheticals was notable for a commander in chief who presided over the creation of a counterterrorism doctrine in which US strikes are supposed to be contemplated only in cases of imminent threat of violent attack. Faced with a terrorist group that is expanding faster than US spy agencies can chart it, the “imminent” threshold appears to have been set aside.

Lisa Monaco, assistant to the president for homeland security and counterterrorism, said in an interview on Saturday that the combination of the speed at which Isil has grown and amassed resources, the number of its fighters holding Western passports and the land it has been able to seize and hold has prompted officials to respond differently than they did to terrorist groups elsewhere. “At least at this stage, it’s a really different type of threat that it poses,” she said.

When asked about the revised estimates of Isil fighters on Friday, White House press secretary Josh Earnest said it indicates “that the group has had some recruitment success after the battlefield advances that they demonstrated back in June, and it reflects some better insight that the intelligence community has been able to gain into the activities” of Isil.

Other factors have also fed US anxiety. Isil’s seizure of large chunks of territory in Iraq and Syria has been particularly unnerving to US officials and agencies still haunted by the extent to which a haven in Afghanistan served as an incubator for Al Qaida and the September 11, 2001, attacks.

In a briefing to reporters this month, Monaco said her top concern about the Islamist group “is its combination of the following: safe havens — plural, money and resources, and manpower.”

US officials have also cited the danger posed by the massive flow of foreign fighters into Syria — including at least 2,000 holding Western passports, enabling them to emerge from the Syrian civil war with Islamist contacts, lethal training and the potential ability to travel throughout Europe and North America unimpeded.

There may also be a significant emotional component. The expanded US strikes were ordered just weeks after most Americans were introduced to Isil on the most brutal terms: through the release of videos in which two US journalists were beheaded by a masked militant speaking with a British accent.

Psychological factors

Some terrorism experts have questioned Obama’s decision to open a multiyear campaign against Isil citing concern that it is being driven more by psychological factors and fear than any hard evidence of an actual threat.

“The American public has come to equate advances in the Middle East by this one group, Isil, with the danger of another 9/11,” said Paul Pillar, a former deputy director of the CIA’s Counterterrorism Centre.

Pillar said that Isil is following a playbook that is in many ways the opposite of Al Qaida’s and that making the group the target of a US-led campaign risks turning its focus toward the United States.

“For them to seize and maintain territory is a major digression from terrorist operations in the West, rather than a facilitation of such operations,” Pillar said.

US strikes can certainly degrade the organisation, but “there will be a revenge factor,” he said. “The killing of the two captive journalists was depicted by the group explicitly as retaliation for strikes that had already occurred.”

Attention to that issue and others has been scarce in the limited Washington debate so far over Isil, a debate that has often been dominated by more dire projections.

“There is no contain policy for Isil,” Secretary of State John Kerry said this month. “They’re an ambitious, avowed genocidal, territorial grabbing, Caliphate-desiring, quasi-state within a regular army. And leaving them in some capacity intact anywhere would leave a cancer in place that will ultimately come back to haunt us.”

Sen Dianne Feinstein, Democratic-California, the chairman of the Senate Intelligence Committee, wrote in a recent op-ed that “the threat Isis poses cannot be overstated.” She went on to describe the group as “the most vicious, well-funded and militant terrorist organisation we have ever seen.”

Although aspects of Kerry’s and Feinstein’s characterisations are accurate, confusion about the group stems to a large degree from the difficulty in extrapolating its danger to the United States from such adjectives.

Isil emerged from the remnants of an Al Qaida affiliate in Iraq that was largely dismantled before US forces left the country in 2011. But the organisation has taken advantage of the chaos in Syria’s civil war and sectarian tensions in Iraq to regroup.

Beyond its swelling ranks of fighters, the organisation has amassed resources at a rapid rate. Its seizure of cities in Iraq this year has enabled it to build an arsenal that includes US-provided weaponry. It also generates an estimated $1 million (Dh3.67 million) a day in revenue from black-market oil sales, kidnappings and other criminal enterprise. Matt Olsen, director of the National Counterterrorism Centre, said recently that Isil has vastly eclipsed Al Qaida in its use of the internet to spread propaganda and entice recruits.

The White House considered that targeting Isil directly could intensify its motivation to strike the United States, Monaco said, which is part of why the president and others have made a point of questioning its religious credentials and overall legitimacy. But she noted that the group has already made its intent to target the country clear.

‘Shown their brutality’

“We conduct that analysis, but they’ve already shown their brutality,” she said.

The threat Isil poses to the region is in some ways more insidious than direct. Its fighters have swept through Sunni areas of Iraq and Syria where local security forces were already weak or disinclined to fight. It would be harder to take on the loyal armies of other countries in the region.

Of greater concern is the flow of foreign fighters, including thousands of Saudis, Jordanians and Tunisians who have probably learned lethal skills in Syria and been drilled in extremist ideology. There have already been demonstrations in support of Isil in Jordan; its flag flutters over some communities in Lebanon; and Saudi Arabia has conducted sweeps to detain dozens of suspected supporters.

For Saudi Arabia and other Gulf countries, there is little incentive to join a military assault on Isil, said Jamal Khashoggi, a prominent Saudi journalist who runs the Al Arab News Channel. “Nobody wants to be in the middle of a bloody sectarian war,” he said. “And if we go into Syria, do we side with the rebels” or Syrian President Bashar Al Assad?

Similar anxieties have spread across Europe, as thousands of Muslims from France, Germany, Britain and other countries have flocked to the conflict in Syria, a country easier for Western militants to reach than Al Qaida havens in Yemen or Pakistan.

French authorities earlier this year arrested an Isil-linked militant who had returned to that country and was discovered with a stockpile of explosives. Another fighter with ties to the group killed four in an attack on a Jewish museum in Belgium. It is unclear whether either had been acting on direction from Isil.

The number of Western fighters in Syria has dwarfed the migrations to other insurgent hot spots in the past. “No more than 50 to 75 American foreign fighters” made it to Afghanistan between 1986 and 2001, said Evan Kohlmann, a terrorism consultant at Flashpoint Partners. “It’s very difficult for law enforcement to monitor this large a number of people.”

US officials have said that about 100 Americans have either travelled to Syria or attempted to and that perhaps a dozen have linked up with Isil. Moner Mohammad Abu Salha — who fought with another extremist group, Jabhat Al Nusra, and was the first known suicide bomber in Syria to come from the United States — recorded a video before his death in May describing how he had eluded FBI surveillance.

Isil’s rivalry with Al Qaida has emerged as another source of worry for US officials. The group severed ties with Al Qaida this year, mocking the older network’s leader, Ayman Al Zawahiri, as too timid and declaring itself the founder of a restored Islamic caliphate.

US counterterrorism officials have warned that a struggle over adherents and resources could lead to competition between the two groups. Nicholas Rasmussen, deputy director of the National Counterterrorism Centre, warned in a recent briefing for reporters that the “competition for primacy in global jihad” could lead to competition in staging spectacular attacks, compounding the danger to the United States. The rivalry, Rasmussen said, makes each side “more unnerving than it might be if judged purely on its own terms.”

And groups such as Al Qaida could recruit some of Isil’s most talented foreign fighters, making them better positioned to strike the West, said Frank Cilluffo, director of George Washington University’s Homeland Security Policy Institute.

Even so, US officials have drawn significant distinctions between Isil and Al Qaida affiliates in Yemen and other countries. The Iraq-based group mixes military and terrorist tactics but is not seen as a patient cultivator of elaborate transnational plots. And there is no indication that its leaders have the technical bomb-making expertise that has enabled Al Qaida in the Arabian Peninsula to devise devices that evaded detection on US-bound aircraft.

“Isil’s ability to carry out complex, large-scale attacks in the West is currently limited,” Rasmussen said in testimony before a Senate committee last week. Al Qaida in the Arabian Peninsula, as the Yemen-based group is known, “remains the Al Qaida affiliate most likely to attempt transnational attacks against the United States.”