Most Americans had never heard of the Khorasan group until this week, when President Barack Obama announced that U.S. airstrikes in Syria had targeted the "seasoned al-Qaeda operatives." U.S. officials said that the Khorasan group was actively plotting to conduct an attack in the United States or Europe.
"We can't say that we definitely disrupted their plots" against the West with the U.S. airstrikes, a senior Obama administration official told one of us, but "there is a decent chance we have" because "their communications are interrupted" and members of the group were killed in the strikes.
The sudden public emergence of the Khorasan group as a threat underlines the fact that the global jihadist movement, which at the time of the 9/11 attacks was largely concentrated in Afghanistan, has morphed and metastasized a great deal since then.
Which raises the question: What are the dimensions of the overall global jihadist threat today?
During the Cold War the U.S. intelligence community knew in detail the size of the Soviet military and the disposition of its forces across Eastern Europe and Russia.
In the long, twilight struggle against al Qaeda, its affiliates, splinter groups, and like-minded organizations -- armed forces that fight without uniforms and often in secret -- such an accounting is harder to do.
A good example of this is the shifting estimates of ISIS's strength. ISIS, which split off from al Qaeda earlier this year, "can muster between 20,000 and 31,500 fighters across Iraq and Syria," a CIA spokesman told CNN last week.
This is as much as three times previous estimate of ISIS's strength; U.S. officials initially estimated ISIS had around 10,000 fighters.
To see if we could come up with some kind of estimate for the total number of militants fighting with jihadist groups around the world, we asked a range of experts to estimate the number of fighters belonging to various al Qaeda-affiliated or like-minded groups. These estimates appear in a report, which we helped to author, that was released this week by the Bipartisan Policy Center's Homeland Security Project, a successor to the 9/11 Commission.
If we tally up the low and high estimates for all these groups, we can begin to have a sense of the total number of jihadist militants that are part of formal organizations around the globe. We found that on the low end, an estimated 85,000 men are fighting in jihadist groups around the world; on the high end, 106,000.
How did we arrive at those numbers?
-- We assess that core al Qaeda, whose members are largely located in the Pakistani tribal regions along the Pakistan-Afghanistan border, has been devastated in recent years by CIA drone strikes and now numbers only in the low hundreds of militants.
-- Al Qaeda in the Arabian Peninsula (AQAP), which operates out of Yemen, poses perhaps the most immediate threat to the U.S. homeland. AQAP had 300 original members in 2010, expanding to around 1,000 by 2012, and membership has remained steady since then, according to Gregory Johnsen, whose book "The Last Refuge: Yemen, al-Qaeda, and America's War in Arabia," is the authoritative history of AQAP.
-- Jabhat al Nusra, the al Qaeda affiliate operating in Syria and northern Iraq, is estimated to be smaller than ISIS, with which it is presently at war. Aaron Zelin, who tracks Nusra for the Washington Institute for Near East Policy, estimates that Nusra has 5,000 to 10,000 fighters. (The Khorasan group is only a small subset of Nusra).
-- Al-Shabaab, al-Qaeda's Somalia affiliate, once controlled much of that country, but it has suffered a number of battlefield losses over the past three years. Ken Munkaus, a professor of political science at Davidson College and a specialist on Somalia, believes that the most reliable estimates for the group put the number of fighters between 3,000 and 5,000.
-- French military intervention in Mali in 2013 largely defeated the forces of al Qaeda in the Islamic Maghreb (AQIM) and its splinter groups, which had taken over half of the country, but AQIM countinues to operate in Algeria, Mali, Mauritania and Niger. AQIM's main appeal is its wealth, as its focus on kidnappings for ransom has brought in an estimated $90 million. Hannah Armstrong, who studies North African militant groups for New America's International Security Program, estimates the total number of AQIM-associated fighters in the Sahel region of North Africa at about 3,000.
Peter Bergen is CNN's national security analyst, a director at the New America Foundation and the author of Manhunt: The Ten-Year Search for bin Laden -- From 9/11 to Abbottabad. Emily Schneider is a research associate at the New America Foundation.