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The Islamic State has been steadily losing territory in its self-declared caliphate in Syria and Iraq, where a U.S. bombing campaign and a host of rival forces chip away at its holdings.
Yet the Brussels bombings again demonstrated the group's potency much farther afield, from terror attacks in Western Europe and North Africa to seizing control of .
"The reality is the Islamic State continues to grow in places like Libya, well beyond Iraq and Syria," , deputy national security adviser during the Bush administration, told NPR's Morning Edition.
"Those capabilities have allowed the Islamic State to present a more direct threat to the West than I think most had assumed," Zarate added.
Territory under ISIS control peaked in the summer of 2014, as it surged into Iraq from its strongholds in eastern Syria. The group declared a caliphate, or Islamic state. Though it received much less attention at the time, ISIS also announced broader goals of expanding its presence in the Muslim world and taking on the West. Many dismissed this as wildly unrealistic.
"ISIS has been waging three parallel campaigns — defending the caliphate, spreading disorder in the wider Middle East and destabilizing the West," said , a counterterrorism analyst at the .
Losing Territory In Iraq And Syria
The U.S. and its allies began bombing ISIS in August 2014, and since then, the group has lost several cities and an estimated 20 to 25 percent of its overall territory, mostly on the periphery. Yet ISIS retains a firm grip on a large patch of territory, including its de facto capital of Raqqa, in northern Syria, and Mosul, the second largest city in Iraq.
Reports by civilians who have fled and defectors who have left ISIS point to increasing hardships. The economy is squeezed, the ultra-strict rules grate on many civilians and ISIS has reportedly cut salaries and benefits to its own fighters.
But the group remains on the offensive in lands far beyond its core holdings.
Read more than a dozen countries have been hit in attacks carried out or inspired by the Islamic State in less than two years, as shown in .
"We are promising the Crusader nations which have aligned themselves against the Islamic State that dark days are coming," ISIS said Tuesday, in claiming responsibility for the Brussels attacks.
The ISIS ambitions raise a key question: Are attacks on multiple fronts likely to strengthen or weaken the group over time?
Extremist Islamist groups have traditionally made an either-or choice: They can control territory locally or carry out terrorist attacks abroad.
ISIS sees itself as a government with a fixed address in Syria and Iraq, and that makes it risky to attack and antagonize other countries. Extremist groups that strike abroad tend to operate underground because they need to guard against the kind of retaliatory strikes that ISIS has faced.
Yet ISIS has tried to turn setbacks at its home base to advantages elsewhere.
Striking Further Afield
ISIS has reportedly told recruits that it's getting harder to reach its territory in Syria and Iraq, and therefore they should go to set up affiliates and carry out attacks in other countries.
Many ISIS attacks, like the double bombing at the airport and subway in Brussels, were well-coordinated operations and not mere lone-wolf attacks that Western security officials have often described as the most likely threat.
"This is an organization that ... still controls territory and is still able to deploy operatives back into the heart of Europe," Zarate said. The group has "now paralyzed another European city. That is something that will persist and continue until ISIS and its safe haven is disrupted."
The U.S. military and European countries are particularly concerned about the several thousand ISIS fighters who have taken control of Sirte, on Libya's Mediterranean coast. This is the strongest ISIS affiliate to date, though it has been spreading across North Africa.
"There is pressure on ISIS, no doubt about it," said Gambhir. "But who is really capable and willing to drive ISIS out of its core terrain? If the answer is no one, then ISIS can still use that terrain to attack in Iraq and Syria, in the broader region and in the West."