Recently, Attorney General Eric Holder described the threat posed by the Islamic State of Iraq and Syria (ISIS) as “more frightening than anything I think I’ve seen as attorney general.”

He has good reason to be afraid. Over 7,000 foreigners from over 70 nations—including the U.S.—have answered ISIS’s call to arms and received training in Iraq and Syria.

After creating a breeding ground for aspiring Islamist militants, ISIS has high hopes for hijacking or terrorizing airplanes by weaponizing cell phones. Thanks to the new partnership between al-Qaeda’s master bomb makers and ISIS, airline passengers are required to prove that their electronic devices are functional. Similar to the underwear bomb plot in 2009, the cell phone bomb threat shows how Islamist bomb makers are becoming increasingly creative in their plans to attack American infrastructure and interests.

Furthermore, American and European ISIS recruits are of key interest due to their passports and easy access to targets in both Europe and the U.S. Americans and Europeans do not raise concern when going through U.S. customs, making it much easier to return home as members of sleeper cells. The FBI recently arrested a Colorado teen on her way to join ISIS.

The formation and expansion of ISIS poses one of the largest threats to U.S. national security over the past few decades. In order to counter this threat, the U.S. should engage in more in-depth counterterrorism operations with European nations. While many European nations have a large stake in the fight against terrorism, a successful campaign against terrorism requires them all to pull their own weight.

While the success of ISIS surprised many, the call for counterterrorism cooperation in Iraq has been prominent for years. The Heritage Foundation’s Middle East expert Jim Phillips emphasized the importance of counterterrorism cooperation:

The Obama Administration has neglected to adequately address the metastasizing threat of al-Qaeda in Iraq. It should work much more closely with the new Iraqi government to combat ISIS and implement a comprehensive national reconciliation strategy to drain away support for the Sunni insurgency and stabilize Iraq.

In order to combat the threat posed by ISIS and foreign fighters returning from a Syrian civil war, policymakers should slow—or, better, stop—sequestration’s hemorrhaging of funding from intelligence programs that help track the traveling patterns and movements of foreign fighters and terrorists around the world.

Travis Klein is currently a member of the Young Leaders Program at The Heritage Foundation. For more information on interning at Heritage, pleaseclick here.