Kurdish Peshmerga fighters assemble at a shrine on Iraq's Mount Sinjar on Friday, December 19. The Kurdish military said that with the help of coalition airstrikes, it has "cleansed" the area of ISIS militants. ISIS has been advancing in Iraq and Syria as it seeks to create an Islamic caliphate in the region.
Editor's note: Peter Bergen is CNN's national security analyst, a vice president at New America Foundation and professor of practice at Arizona State University. He is the author of "Manhunt: The Ten-Year Search for bin Laden -- From 9/11 to Abbottabad." Emily Schneider is a research associate at New America.
(CNN) -- In the many media stories about the Islamic State of Iraq and Syria, much of the focus has rightly been on the thousands of foreign fighters ISIS has attracted, its brutal tactics and its robust social media presence.
But an arguably even more important development has not received the attention it deserves: the group's widening influence across the Muslim world, driven by the numerous terrorist and insurgent organizations that have recently sworn loyalty to it.
In the past six months, ISIS has drawn into its fold some dozen groups from Algeria to Pakistan. Al Qaeda, in contrast, had been in existence for a decade before it recruited its first affiliate, Egypt's Jihad Group, in 1998. And, in its 2½-decade existence, al Qaeda has only manged to add some half dozen affiliates, one of which was al Qaeda in Iraq, the parent organization of ISIS that has now split off from the core al Qaeda organization.
Indeed, just this week, an ISIS delegation met with leaders of a splinter group of the Pakistani Taliban to talk about how to unify Pakistani militants, The Associated Press reported.
Also this month, Ansar Beit al-Maqdis, one of the most violent militant groups in Egypt, pledged allegiance to ISIS, The New York Times reported. ABM is believed to have been responsible for an attack on a police checkpoint near Gaza last month that killed 30 Egyptian soldiers.
And ISIS has continued to expand its geographical reach. Three other groups -- al-Mujahidin in the Arabian Peninsula, al-Mujahidin in Libya and al-Mujahidin in Yemen -- also recorded statements of allegiance to ISIS in November, which ISIS broadcast online. Meanwhile, ISIS now controls the eastern Libyan city of Derna, not far from the Egyptian border.
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And last month, six Pakistani Taliban leaders reportedly swore allegiance to ISIS in an audio message, although Shahidullah Shahid, the Pakistani Taliban's official spokesman, said he was speaking for himself and five other Taliban leaders in the message, not for the rest of the Pakistani Taliban.
The Taliban reacted by firing Shahid, but a senior Taliban official told the BBC that "he was the most important of the five who have left us" and said that the leaders had defected because they were unhappy with senior Taliban leaders.
Among other groups that have pledged allegiance to ISIS is Jund al-Khalifa (Soldiers of the Caliphate), a terrorist group operating in Algeria. Previously, the group was part of al Qaeda's North African branch, al Qaeda in the Islamic Maghreb, until the group split from AQIM because it was "deviating from the true path." And just 10 days after proclaiming its allegiance to ISIS, Jund al-Khalifa beheaded Herve Gourdel, a 55-year-old French hiker they had taken hostage. (Beheading Western hostages is, of course, one of ISIS' signature tactics).
Inside Syria and Iraq, ISIS has also gathered up smaller militant groups. For example, Ansar al-Islam, a former al Qaeda-linked group, is now cooperating with ISIS. Also, Jaysh Mohammed (Army of Mohammed) used to work to repel U.S. forces from Iraq and has now joined the ranks of ISIS.
Finally, ISIS and the al Qaeda-aligned Nusra Front had previously been at war with each other in Syria for much of this past year. But this month, representatives from the two groups met in Syria and agreed to stop fighting each other and focus on fighting their common enemies, sources told The Associated Press. This brings together the two most effective Sunni militant groups fighting in Syria.
Why are all of these militant groups suddenly pledging their allegiance to ISIS or aligning themselves with the group, and in many cases giving up former affiliations with al Qaeda's core group to do so?
The answer is simple: ISIS is successful.
In many ways it really is the "Islamic State of Iraq and Syria" that it claims to be, because it has taken large portions of both countries from government control, replacing local administrators with ISIS governors who distribute social services. ISIS is also winning critical cities and key points of infrastructure, including dams and oil wells, and that means the group has substantial funding and resources at its disposal.
Partnering with ISIS makes sense from an economic perspective for many of these organizations, especially the smaller militant groups operating inside Syria and Iraq. For other militant groups around the Muslim world, it allows for a cross-pollination of ideas and training through a sort of jihadist exchange program, where some groups send members into Syria or Iraq to learn from ISIS.
ISIS controls territory in a 400-mile swath from Aleppo in the west of Syria to Fallujah in the east, near the Iraqi capital, Baghdad, while its dozen or so affiliated organizations stretch from the coast of North Africa to the mountains of the Hindu Kush.
The fact is that al Qaeda's core organization can only dream of such success.
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A Peshmerga fighter looks down at the body of an alleged ISIS fighter in Zummar, Iraq, on Thursday, December 18.
A Kurdish fighter stands next to a destroyed armored vehicle in northern Iraq on December 18. The vehicle was destroyed by an improvised explosive device placed by ISIS militants.
Peshmerga fighters stop to check a vehicle in Zummar on December 18 as they continue to battle ISIS fighters near the border with Syria.
Pro-Iraqi government forces guard a shrine in Balad, Iraq, on Monday, December 15.
A Yazidi woman displaced by ISIS militants tends to a fire Wednesday, December 10, at a shelter in Dohuk, Iraq.
A Kurdish child from the Kobani, Syria, area holds laundry at a refugee camp in Suruc, Turkey, on Monday, November 17. Tens of thousands of people have fled Kobani, known in Arabic as Ayn al-Arab, to escape ISIS.
Smoke rises from Kobani following airstrikes on November 17. The United States and several Arab nations have been bombing ISIS targets to take out the group's ability to command, train and resupply its fighters.
A Kurdish child from the Kobani area holds on to a fence at a refugee camp in Suruc on Sunday, November 16.
People in Suruc watch smoke rise near the Syrian border during clashes between ISIS members and armed groups on Thursday, November 13.
A bomb (upper left) falls on an ISIS position in Kobani during an airstrike by the U.S.-led coalition on Saturday, November 8.
Iraqi military forces take up position in Jurf al-Sakhar, Iraq, on November 8.
Fire and smoke rise from Kobani following airstrikes against ISIS on Thursday, October 30.
Iraqi special forces search a house in Jurf al-Sakhar on October 30 after retaking the area from ISIS.
Smoke rises during fighting in Kobani on Monday, October 27.
ISIS militants stand near the site of an airstrike near the Turkey-Syria border on Thursday, October 23.
An explosion rocks Kobani during a reported car-bomb attack by ISIS militants on Monday, October 20.
People watch Kobani from a hill near the Turkey-Syria border on October 20.
Kurdish fighters walk to positions as they fight ISIS forces in Kobani on Sunday, October 19.
A U.S. Air Force plane flies above Kobani on Saturday, October 18.
Heavy smoke rises in Kobani following an airstrike by the U.S.-led coalition on Saturday, October 18.
Cundi Minaz, a female Kurdish fighter, is buried in a cemetery in the southeastern Turkish town of Suruc on Tuesday, October 14. Minaz was reportedly killed during clashes with ISIS militants in nearby Kobani.
Turkish police officers secure a basketball stadium in Suruc on October 14. Some Syrian Kurds have been held there since crossing from Syria into Turkey.
Kiymet Ergun, a Syrian Kurd, celebrates in Mursitpinar, Turkey, after an airstrike by the U.S.-led coalition in Kobani on Monday, October 13.
Smoke rises from Kobani on Sunday, October 12.
Syrian Kurds from Kobani stand outside the grounds of a refugee camp in Suruc on Saturday, October 11.
Alleged ISIS militants stand next to an ISIS flag atop a hill in Kobani on Monday, October 6.
In this photo released by the U.S. Air Force on Saturday, October 4, a U.S. Navy jet is refueled in Iraqi airspace after conducting an airstrike against ISIS militants.
A Kurdish Peshmerga soldier who was wounded in a battle with ISIS is wheeled to the Zakho Emergency Hospital in Duhuk on Tuesday, September 30.
Iraqi Shiite militiamen aim their weapons during clashes with ISIS militants in Jurf al-Sakhar on Sunday, September 28.
Syrian Kurds wait near a border crossing in Suruc as they wait to return to their homes in Kobani on Sunday, September 28.
Syrian Kurds wait behind border fences to cross into Suruc on September 28.
Tomahawk missiles, intended for ISIS targets in Syria, fly above the Persian Gulf after being fired by the USS Philippine Sea in this image released by the U.S. Navy on Tuesday, September 23.
Turkish Kurds clash with Turkish security forces during a protest near Suruc on Monday, September 22. According to 
Syrian Kurds fleeing ISIS militants wait behind a fence in Suruc on Sunday, September 21.
A elderly man is carried after crossing the Syria-Turkey border near Suruc on Saturday, September 20.
A Kurdish Peshmerga fighter launches mortar shells toward ISIS militants in Zummar on Monday, September 15.
An ISIS flag flies on the other side of a bridge at the front line of fighting between ISIS and Kurdish Peshmerga fighters in Rashad, Iraq, on Thursday, September 11.
U.S. Secretary of State John Kerry reads on a flight en route to Iraq on Wednesday, September 10. Kerry traveled to the Mideast to discuss ways to bolster the stability of the new Iraqi government and combat ISIS.
Kurdish Peshmerga fighters fire at ISIS militant positions from their position on the top of Mount Zardak, east of Mosul, Iraq, on Tuesday, September 9.
An Iraqi fighter jet flies over Amerli, Iraq, on September 3. Amerli had been under siege by ISIS militants.
Iraqi volunteer fighters celebrate breaking the Amerli siege on Monday, September 1. ISIS militants had surrounded Amerli, 70 miles north of Baquba, Iraq, since mid-June.
Kurdish Peshmerga forces stand guard at their position in the Omar Khaled village west of Mosul on Sunday, August 24.
Kurdish Peshmergas fight to regain control of the town of Celavle, in Iraq's Diyala province, on August 24.
Peshmerga fighters stand guard at Mosul Dam in northern Iraq on Thursday, August 21. With the help of U.S. military airstrikes, Kurdish and Iraqi forces
Displaced Iraqis receive clothes from a charity at a refugee camp near Feeshkhabour, Iraq, on Tuesday, August 19.
A fighter with Kurdish Peshmerga forces battles ISIS militants near Mosul on Monday, August 18.
Peshmerga fighters inspect the remains of a car that reportedly belonged to ISIS militants and was targeted by a U.S. airstrike in the village of Baqufa, north of Mosul, on August 18.
Kurdish Peshmerga fighters fire at ISIS in Khazair, Iraq, on Thursday, August 14.
Volunteers of the Iraqi Red Crescent Society unload boxes of goods before distributing them August 14 to families who fled from ISIS.
From the flight deck of the USS George H.W. Bush, which is in the Persian Gulf, two U.S. fighter jets take off for a mission in Iraq on Monday, August 11. U.S. President Barack Obama authorized airstrikes against Islamic militants and food drops for Iraqis who are trapped by the militants.
Aziza Hamid, a 15-year-old Iraqi girl, cries for her father while she and some other Yazidi people are flown to safety Monday, August 11, after a dramatic rescue operation at Iraq's Mount Sinjar. A CNN crew was on the flight, which took diapers, milk, water and food to the site where as many as 70,000 people were trapped by ISIS. But only a few of them were able to fly back on the helicopter with the Iraqi Air Force and Kurdish Peshmerga fighters.
Thousands of Yazidis are escorted to safety by Kurdish Peshmerga forces and a People's Protection Unit in Mosul on Saturday, August 9.
Iraqi Shiite volunteers who have joined government forces to fight ISIS take part in a training session near Basra, Iraq, on Thursday, August 7.
Thousands of Yazidi and Christian people flee Mosul on Wednesday, August 6, after the latest wave of ISIS advances.
A Baiji oil refinery burns after an alleged ISIS attack in northern Selahaddin, Iraq, on Thursday, July 31.
A Syrian rebel fighter lies on a stretcher at a makeshift hospital in Douma, Syria, on Wednesday, July 9. He was reportedly injured while fighting ISIS militants.
Iraqis who fled fighting in the cities of Mosul and Tal Afar try to enter a temporary displacement camp in Khazair on Wednesday, July 2.
Peshmerga fighters check cars at the entrance of a temporary displacement camp in Khazair on Thursday, June 26.
Kurdish Peshmerga take their positions behind a wall on the front line of the conflict with ISIS militants in Tuz Khormato, Iraq, on Wednesday, June 25.
Peshmerga fighters clean their weapons at a base in Tuz Khormato on June 25.
New army recruits gather in Najaf, Iraq, on Wednesday, June 18, following a call for Iraqis to take up arms against Islamic militant fighters.
An MH-60R Sea Hawk helicopter lands on the aircraft carrier USS George H.W. Bush in the Persian Gulf on Tuesday, June 17.
Newly recruited Iraqi volunteer fighters take part in a training session in Karbala, Iraq, on June 17.
Members of ISIS prepare to execute soldiers from Iraq's security forces in this image, one of many reportedly posted by the militant group online. CNN cannot independently confirm the authenticity of the images.
Iraqi men chant slogans outside of an army recruiting center to volunteer for military service Thursday, June 12, in Baghdad.
Kurdish Peshmerga forces, along with Iraqi special forces, deploy their troops and armored vehicles outside of Kirkuk, Iraq, on June 12.
Children stand next to a burnt vehicle during clashes between Iraqi security forces and ISIS militants in Mosul on Tuesday, June 10.
Civilians from Mosul escape to a refugee camp near Irbil, Iraq, on June 10.
