Analysts say they expect to see an uptick in terror attacks inspired by the Islamic State group in the wake of last weekend's deadly assault near London Bridge – a position starkly at odds with the pronouncements of U.S. military leaders who have long predicted the group's ability to inspire violence would diminish with the size of its so-called caliphate.

"As perverse as it sounds, the ISIS brand and its violent ideology still resonates strongly with a global following not deterred by its battlefield losses," says Martin Reardon, a Marine Corps and FBI veteran with deep experience in counterterrorism and intelligence operations, now a senior vice president at The Soufan Group.

Reardon made the remark this week after a spate of attacks, including three in the U.K. in as many months, carried out by perpetrators in the name of the Islamic State group but who may have had no previous ties to the extremist network, also known as ISIS. The attacks are noteworthy because they follow a series of military losses for the group – losses that many had predicted would erode its ability to inspire new acts of terror as its fighters increasingly became associated with defeats.

The prevailing belief was that the group relied on its so-called caliphate – at the center of much of its apocalyptic ideology and promoted through the unprecedented use of social media – to galvanize the faithful. The extremists initially encouraged followers to travel to the area of Iraq and Syria under its control after it rose to power in the summer of 2014 only to later direct converts who could not reach the region to mount attacks where they lived.

Marine Gen. Joseph Dunford, the chairman of the Joint Chiefs of Staff, has made clear that the the fight against the Islamic State group, militarily and ideologically, extends beyond Iraq and Syria. But he has also frequently emphasized the importance of taking back ground there as a way to undermine the extremists' narrative – eroding the group symbolically as well as territorially as a way of countering the propaganda it employed to win over impressionable foreigners.

Army Col. Ryan Dillon, a spokesman for the U.S.-led military coalition fighting the Islamic State group, as recently as this week linked the outcomes in describing the undercutting of the group's ability inside Iraq and Syria to make money off stolen oil or other regional resources, tax the people under its control or exploit border crossings.

"We are degrading their ability not just to conduct just battlefield operations but to plan, inspire and direct terror attacks," he said in a phone interview from his headquarters in Baghdad.

The logic, however, is increasingly being tested. Those with knowledge of the group's methods now say that obvious and undeniable losses and the sieges of its capitals of Raqqa in Syria and Mosul in Iraq have had little effect in curtailing the willingness of sympathizers to act out on the group's behalf as they did in 2015 in San Bernardino, California, or last year in Nice, France.

"Their messaging is still very popular," says Otso Iho, a senior analyst with the London-based Jane's Terrorism and Insurgency Centre. "We haven't seen a huge amount of signs suggesting that ISIS has lost that stature of a household brand yet."

Iho said in an analysis of the most recent London attack that the increased frequency of terror incidents also indicates a "substantial increase in the pool of potential attackers," a concern that military and political leaders have shared since noting that battlefield success against the Islamic State group has caused the extremists to shift tactics. Former President Barack Obama said as much in August, when he warned that the group appeared to be "encouraging high-profile terrorist attacks, even in the United States."

"If Raqqa, say, falls in five months, that doesn't spell the end of ISIS," Iho says. "That doesn't spell the end of its influence, particularly as we see foreign fighters leave Iraq and Syria, and some of whom will be able to embed themselves in local networks in Europe."

Islamic State group fighters have been connected to terror attacks in France, Belgium and just Wednesday in Iran. Indeed, top military and intelligence planners – including Dunford – have said that the ultimate conflict against the extremists would not end on the battlefield in Iraq and Syria.

"ISIS is unlikely to announce that it is ending its self-declared caliphate even if it loses overt control of its de facto capitals in Mosul, Iraq and Ar Raqqah, Syria and the majority of the populated areas it once controlled in Iraq and Syria," the Office of the Director of National Intelligence said in an assessment it released in May.

"The number of ISIS foreign fighters leaving Iraq and Syria might increase," it stated. "Increasing departures would very likely prompt additional would-be fighters to look for new battlefields or return to their home countries to conduct or support external operations."

While the problem of returning fighters – what former FBI Director James Comey termed the "terrorist diaspora" – is being experienced across the globe, most expected the group to lose stature among potential new recruits as it shed soldiers.

Comey told an audience at the University of Texas at Austin in March that the timelessness of the internet meant the agency was still dealing with the "lingering phenomenon" of people who are searching for meaning finding it in the online propaganda of the Islamic State group – what he called "on the spectrum between consuming the poison and acting on the poison."

But he said the group's ability to "inspire or enable or direct people in the United States to engage in acts of violence" had waned precipitously.

"Their efforts to use the power of social media to get those who are not going to travel to kill where they are peaked in the spring of 2015," Comey said. "And that's because they invested for almost a year in a social media brand and constantly pushing out these images and this siren song: Come or kill, come or kill."

As the attacks in London and Paris this week demonstrated, the siren song continues to compel. And some say it shows little sign of losing its appeal.