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 the New America Foundation and the author of "Manhunt: The Ten-Year Search for bin Laden -- From 9/11 to Abbottabad." David Sterman is a research associate at the New America Foundation.
(CNN) -- U.S. officials are claiming that the terrorist group Islamic State in Iraq and Syria, or ISIS, is "now a credible alternative to al Qaeda."
But what does that really mean in terms of ISIS' potential threat to the United States? After all, al Qaeda hasn't pulled off a successful attack in the States since 9/11, or indeed anywhere in the West since the London transportation bombings in 2005.
This month, Sen. John McCain, R-Arizona, criticized the airstrikes in Iraq ordered by President Barack Obama directed at ISIS as too limited, telling CNN's Candy Crowley, "That is simply a very narrow and focused approach to a problem which is metastasizing as we speak. Candy, there was a guy a month ago that was in Syria, went back to the United States, came back and blew himself up. We're tracking 100 Americans who are over there now fighting for ISIS. ISIS is attracting extreme elements from all over the world, much less the Arab world. And what have we done?"
The case McCain alluded to was that of Moner Mohammad Abu-Salha, who grew up in Vero Beach, Florida, and who conducted a suicide bombing in Syria in May on behalf of the Nusra Front, al Qaeda's Syrian affiliate. According to The New York Times, Abu-Salha had returned to the United States after being trained by Nusra and then went back to Syria to conduct the suicide operation in which he died.
McCain asserted on CNN that 100 Americans were fighting with ISIS. In fact, according to U.S. officials, 100 is the total number of Americans believed to have fought or attempted to have fought with any of the many Syrian insurgent groups, some of which are more militant than others, and some of which are even aligned with the United States.
According to a count by the New America Foundation, eight people from the United States have been indicted with crimes related to trying to join ISIS or the Nusra Front. (By contrast, some 240 U.S. citizens and residents have been indicted or charged with some kind of jihadist terrorist crime since 9/11.)
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Some of the Nusra Front cases are far from threatening. On April 19, 2013, Abdella Tounisi, an 18-year-old American citizen from Aurora, Illinois, was arrested and charged with attempting to provide material support to Nusra. However, he was caught in a sting operation and described his fighting skills thusly: "Concerning my fighting skills, to be honest, I do not have any." Tounisi pleaded not guilty and awaits trial.
Other cases appear more serious. In December, Sinh Vinh Ngo Nguyen, a U.S. citizen from Southern California, pleaded guilty to a charge of attempting to provide material support to al Qaeda. Between December 2012 and April 2013, Nguyen had traveled to Syria, where, he stated, he fought alongside the Nusra Front. On his return, Nguyen discussed with an informant his intent to participate further in jihad.
In August 2013, Gufran Mohammed, a naturalized American citizen living in Saudi Arabia, was charged with attempting to provide material support to the Nusra Front in Syria, by facilitating the recruitment of experienced fighters from al Qaeda's Somali affiliate to Syria.
He pleaded guilty last month.
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Yet so far no U.S. citizen involved in fighting or supporting the Nusra Front or ISIS has been charged with plotting to conduct an attack inside the United States despite the fact the war in Syria is now in its fourth year and the war in Iraq is in its 11th year. Indeed, some Americans who have traveled to Syria have ended up dead apparently because they have no combat experience to speak of; for instance, Nicole Mansfield from Flint, Michigan, was killed in Syria last year by forces loyal to the Syrian dictator Bashar al-Assad.
Further, ISIS' predecessor, al Qaeda in Iraq, never tried to conduct an attack on the American homeland, although it did bomb three American hotels in Jordan in 2005.
And it's also worth noting that in none of the successful terrorist attacks in the States since 9/11, such as the Boston Marathon bombings last year or Maj. Nidal Hasan's massacre at Fort Hood, Texas, in 2009, did any of the convicted or alleged perpetrators receive training overseas.
Returning foreign fighters from the Syrian conflict pose a far greater threat to Europe, which has contributed a much larger number of foreign fighters to the conflict than the United States, including an estimated 700 from France, 450 from the United Kingdom and 270 from Germany.
Unlike in the United States, European countries have reported specific terrorist plots tied to returning Syrian fighters. Mehdi Nemmouche, a suspect in the May 24 shootings at a Jewish museum in Brussels, Belgium, that killed four people, spent about a year with jihadist fighters in Syria, according to the Paris prosecutor in the case. But Nemmouche's case is the only instance of lethal violence by a returning Syrian fighter in the West.
Still, the United States must consider European foreign fighters returning from Syria as more than a European problem because many of those returning are from countries that participate in the U.S. visa waiver program and can enter the States without a visa.
Moreover, experienced al Qaeda operators are present in Syria. As one senior U.S. intelligence official put it to us, these are veteran members "with strong resumes and full Rolodexes." The wars in Syria and Iraq allow such longtime fighters to interact with members of other al Qaeda affiliates. For example, in July, the United States adopted enhanced security measures at airports based on intelligence that bomb-makers from al Qaeda in the Arabian Peninsula were sharing their expertise in making bombs capable of evading airport security with members of the Syrian Nusra Front.
Despite these dangers, however, the threat to the United States from foreign fighters in Syria and Iraq remains only a potential threat.
The administration's airstrikes in Iraq are properly focused upon the more imminent threats to U.S. government employees and American citizens in the Kurdish city of Irbil who are threatened by ISIS advances and the humanitarian catastrophe befalling the Yazidi population in areas controlled by the militant forces.
The last time there was a similar exodus of American citizens and residents to an overseas holy war was to Somalia following the U.S.-backed invasion of Somalia by Ethiopian forces in 2006. More than 40 Americans subsequently went to Somalia to fight with Al-Shabaab, an al Qaeda-affiliated group.
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Just as is the case today in Syria, for a good number of the Americans who went to fight in Somalia it was a one-way ticket because 15 of the 40 or so American volunteers died there either as suicide attackers or on the battlefield.
In 2011, Rep. Peter King, R-New York, then-chairman of the House Homeland Security Committee, warned of Americans fighting in Somalia. "With a large group of Muslim-Americans willing to die as 'martyrs' and a strong operational partnership with al Qaeda leaders in Pakistan and in Yemen, al-Shabaab now has more capability than ever to strike the U.S. homeland."
As it turned out, those Americans who returned from the Somali jihad did not attempt or carry out any kind of terrorist attack in the States.
Now King is back at it again, telling NBC last week, "ISIS is a direct threat to the United States of America. ... They are more powerful now than al Qaeda was on 9/11."
ISIS is surely a major problem for Iraq, and its tactics and strategy are abhorrent, as demonstrated by the beheading of American journalist James Foley, its use of crucifixions and its genocidal attacks on the small Yazidi minority. But that doesn't mean it is a serious threat to the American homeland.
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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.
