Experts are keeping an eye on several locations where ISIS may recognize formal affiliates in the future. Abu Sayyaf, a militant group in the Philippines that has already vowed allegiance to ISIS and carried out multiple abductions of foreign nationals, is seen as a likely candidate, as is an affiliate in Bangladesh, where ISIS-linked terrorists claimed credit for bombings of Shiite mosques last fall and a string of recent hacking deaths, including an academic and a Hindu tailor.

Other possibilities include Tunisia, which has exported the highest number of foreign fighters to ISIS of any country, and Somalia. ISIS claimed responsibility for two high-profile mass killings in Tunisia last year, both of which targeted tourist spots. In Somalia, ISIS has also been trying to attract enough defectors from the Al Qaeda-linked group Al Shabaab to declare an affiliate.

Beyond its affiliates, ISIS has absorbed an unprecedented number of foreign fighters from dozens of countries in the Middle East, Asia, Africa and Europe. In its propaganda, ISIS advertises its ability to attract foreign recruits as a way to project the reach of the caliphate. While the wilayats serve as outposts for recruiting, carrying out attacks and gaining territory, the foreign recruits can be used to export terrorism.

“Tackling ISIS affiliates around the world must start with weakening or eradicating the organization in Iraq and Syria first.”

Authorities believe that a cell of French and Belgian ISIS recruits carried out the November 2015 attacks in Paris that killed 130 people, as well as the March 2016 suicide bombings in Brussels that claimed at least 30 lives.

Most concerning, officials say, is that the Paris and Brussels attackers represent only a small fraction of foreign fighters that have left Europe to join ISIS. The International Center for Counter-Terrorism -- The Hague estimated in April that between 3,900 and 4,290 fighters had traveled from Western Europe to join ISIS or other groups in Iraq and Syria.

Fighters from Europe in Iraq and Syria France: >900

Germany: 720 - 760

United Kingdom: 700 - 760

Belgium: 420 - 516

Sweden: 250 - 300

Austria: 230 - 300

The Netherlands: 220

Spain: 120 - 139

Denmark: 125

Italy: 87 Source: International Centre for Counter-Terrorism - The Hague

The challenge authorities have had is tracking how many come back, and how many returnees are likely to carry out attacks. The Hague estimates that 30 percent of foreign fighters from Western Europe returned to their home countries, but it points out that "Not all foreign fighters are terrorists, and not all terrorists are foreign fighters."

European and Iraqi intelligence officials told the Associated Press following the Brussels attack that an estimated 400 to 600 ISIS fighters were trained specifically to carry out attacks in Europe.

ISIS has used its online magazine, Dabiq, to articulate the goal of "eliminating the grayzone" by using attacks to turn Western societies against their Muslim populations, thereby expanding the pool from which it can recruit.

Clint Watts, Fox Fellow at the Foreign Policy Research Institute, is skeptical of ISIS's capacity for such a "grand design." He says while European recruits may be driven by personal vengeance, for ISIS, such terrorist attacks are retaliation for Europe's bombing campaign in Iraq and Syria. Watts said these attacks also give ISIS a moment in the spotlight, even as it loses territory in Syria and Iraq.

"You can do five attacks in North Africa, and you get a fraction of the media coverage you get for 30 people in Brussels."

As with the expansion-through-affiliates strategy, it all comes back to increasing the sphere of ISIS's seeming influence, said Khatib of Chatham House.

"ISIS tells its followers around the world to engage in opportunistic attacks whenever and wherever they can," she said. "That's part of the drive to present the organization as having global influence."

It's also why, Khatib said, one key to stopping the expansion of ISIS is for the West and its allies in the Middle East to eliminate the impression that ISIS central is "seemingly robust."

"Tackling ISIS affiliates around the world must start with weakening or eradicating the organization in Iraq and Syria first."