In the wake of the U.S. withdrawal, a resurgent al-Qaeda in Iraq, allied with jihadist fighters in neighboring Syria, constitutes a considerable threat to regional security, explains this Backgrounder.

Some experts believe al-Shabaab is at its weakest point in years following an African-led counterinsurgency campaign, but others warn of the group's resiliency in an unstable Somalia.

The expansion of North Africa's AQIM into Mali and links to attacks against the U.S. consulate in Libya have spurred fears about the group's threat to the region, explains this CFR backgrounder.

The People's Mujahedin of Iran, or MEK, is a militant Iranian opposition group and a recognized terrorist organization, according to the U.S. State Department. This Backgrounder examines the group's history and the politics surrounding its relocation from Iraq.

NATO has been the foundation of transatlantic security for more than sixty years, but despite its longevity, critics question whether the alliance can stay relevant in the face of emerging threats, limited funding, and debates over its mission.

Targeted killings have become a central component of U.S. counterterrorism operations around the globe. Despite pointed criticism over transparency and accountability issues, analysts say the controversial practice seems likely to expand in the future.

President Obama vowed in January 2009 to close the prison camp at Guantanamo Bay. Two years later, the White House continues to face challenges to that promise, leaving critics to suggest the facility will remain open for the foreseeable future.

Al-Qaeda in the Arabian Peninsula has become the most dangerous al-Qaeda affiliate and is attempting to exploit the ongoing political unrest in Yemen. This Backgrounder profiles the group and looks at U.S. counterterrorism operations against its members.

The U.S. government designates Iran as the "most active state sponsor of terrorism," which feeds concerns about Iran's growing nuclear program.

Ten years after being toppled from power in Kabul, the Taliban remains resilient in Pakistan and Afghanistan, and complicates U.S. efforts to wind down the Afghan war.

Pakistan has emerged as a terrorist sanctuary for some of the world's most violent groups, including al-Qaeda, the Taliban, and homegrown militants, that threaten the stability of Pakistan as well as the region.

A profile of the international terrorist network that the United States has singled out as the most serious threat to U.S. security.

Ayman al-Zawahiri, now America's most wanted terrorist, took over as the head of al-Qaeda in June 2011 after the killing of erstwhile head Osama bin Laden.

Pakistan's intelligence agency, the ISI, faces persistent accusations of links to terrorism, despite repeated denials.

Violent acts by homegrown militant extremists in the U.S. have declined, but "lone wolf" attacks are on the rise. The post-9/11 legal and political landscape poses new challenges to law enforcement authorities seeking to prevent such attacks.

An increase in terror incidents involving Islamic radicals who are U.S. citizens is vexing law enforcement officials and posing new questions about the roots of their radicalization.

A profile of Hassan Nasrallah, the secretary-general of Hezbollah.

A profile of the Lebanese terrorist organization.

Chechnya has been plagued by two wars and an ongoing insurgency since the fall of the Soviet Union. In recent years, Chechen militants have escalated attacks in the North Caucasus and revived bombings in Moscow.