A joint report between two Washington, D.C.-based think tanks concludes that the US is dangerously underestimating a jihadist group that could become even more of a threat to the long-term security of the country than ISIS.

The Institute for the Study of War and the American Enterprise Institute released its report last week. A group of experts, some of whom were involved in planning the 2007 surge of US troops in Iraq, met over multiple weeks to create the report.

The report said Al Qaeda's affiliate in Syria, Jabhat al-Nusra, posed "one of the most significant long-term threats" of any jihadist group.

"This Al Qaeda affiliate has established an expansive network of partnerships with local opposition groups that have grown either dependent on or fiercely loyal to the organization," the report said. "Its defeat and destruction must be one of the highest priorities of any strategy to defend the United States and Europe from Al Qaeda attacks."

While the US's strategy in the Middle East is heavily focused on ISIS, which is also known as the Islamic State, ISIL, or Daesh, Jabhat al-Nusra, which is also known as the Nusra Front, is spreading its influence through groups that oppose the regime of Syrian President Bashar Assad.

Fighters with whom the US partners in Syria have previously been told they must focus on battling ISIS and refrain from attack Assad's troops. But ISW and AEI pointed out that deposing Assad, a brutal leader who has been accused of massacring his own citizens, is the top priority for many rebels.

In that case, they'll align with the groups with the best funding and equipment that allow them the freedom to fight both ISIS and Assad. In many areas, that group is Jabhat al-Nusra.

"Jabhat al-Nusra has weakened the moderate opposition and penetrated other Sunni opposition groups in Syria so thoroughly that it is poised to benefit the most from the destruction of ISIS and the fall or transition of the Assad regime," the report said.

"The likeliest outcome of the current strategy in Syria, if it succeeds, is the de facto establishment and ultimate declaration of a Jabhat al-Nusra emirate in Syria that has the backing of a wide range of non-al-Qaeda fighting forces and population groups," it continued.

ISW and AEI predicted that Jabhat al-Nusra could then become a key affiliate for the global Al Qaeda terrorist network that focuses on attacking the West.

So far, it appears that Jabhat al-Nusra has been focused mostly on fighting in Syria. But that could be part of a strategy to avoid scrutiny from Western officials.

"The fact that the US is focused so exclusively on ISIS means that we are ignoring a threat that is as great," Kimberly Kagan, the founder and president of ISW and one of the authors of the report, told Business Insider.

Jabhat al-Nusra is playing a "long game," Kagan said.

"ISIS is in fact overt about its presence and Nusra is covert about its presence," she said. "Nusra's covert presence means the US hasn't focused enough on its presence."

She added: "Al Qaeda's senior leaders have had a deliberate strategy of where they host cells that are planning deliberate attacks against the West at any given moment. Because the US has deliberately targeted Al Qaeda on the basis of whether or not there are attack cells focused on the West, Al Qaeda has tried to minimize the footprint of these cells in areas where it actually wishes to see long-term success. Syria is the top priority for Al Qaeda."

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Other experts, however, have characterized the potential threat from Jabhat al-Nusra in less dire terms.

Fred Hof, a senior fellow at the Atlantic Council who was a special adviser for transition in Syria under Hillary Clinton, then the secretary of state, agreed that Nusra's resources had attracted many anti-Assad rebels to the group's ranks. But he contended that these fighters weren't very interested in broader operations.

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