Demonstrators chant in favor of the Islamic State and carry the group's flags in Mosul, Iraq, in June. With videos, Internet magazines and social media, the group has effectively recruited throughout the world. AP hide caption

itoggle caption AP

Iraqi soldiers walk in Jurf al-Sakhr, south of the capital Baghdad, on Monday after Iraqi military forces retook the area from Islamic State militants. Iraqi forces, supported by U.S. airstrikes, have made limited gains in recent months, but critics are questioning whether the U.S. strategy is likely to succeed. Haidar Mohammed Ali/AFP/Getty Images hide caption

itoggle caption Haidar Mohammed Ali/AFP/Getty Images

An explosion following an airstrike is seen in the Syrian town of Kobani from near the Mursitpinar border crossing in the southeastern town of Suruc, in Turkey's Sanliurfa province, on Wednesday. Yannis Behrakis/Reuters/Landov hide caption

itoggle caption Yannis Behrakis/Reuters/Landov

Three female members of Turkey's Kurdistan Workers' Party (PKK) and an Iraqi Kurdish peshmerga fighter stand near the front line in Makhmur, in northern Iraq, on Aug. 9. The Turkish and Iraqi Kurds have been fighting together against the Islamic State. Safin Hamed/AFP/Getty Images hide caption

itoggle caption Safin Hamed/AFP/Getty Images

The site at Gobekli Tepe, or "Potbelly Hill," on the Urfa plain in southeastern Turkey is believed by some to be the world's first place of worship. This would upend the conventional thought that religion developed as a byproduct of human settlements. Vincent J. Musi/National Geographic Society/Corbis hide caption

itoggle caption Vincent J. Musi/National Geographic Society/Corbis

A Free Syrian Army fighter runs after attacking a tank with a rocket-propelled grenade during fighting in Aleppo, Syria, in September 2012. The rebels say they are willing to take on the Islamic State, but need more weapons. Manu Brabo/AP hide caption

itoggle caption Manu Brabo/AP

Thick smoke rises following an airstrike by the U.S.-led coalition in Kobani, Syria, while fighting continued between Syrian Kurds and the militants of the self-proclaimed Islamic State, as seen from Mursitpinar on the outskirts of Suruc, at the Turkey-Syria border on Wednesday. Lefteris Pitarakis/AP hide caption

itoggle caption Lefteris Pitarakis/AP

Abdul-Rahman Kassig, who was formerly known as Peter, is shown with a truck filled with aid supplies for Syrian refugees. The American aid worker was seized by the Islamic State in October 2013 and was killed by the group, which released a video Sunday of his beheading. Courtesy of Kassig Family hide caption

itoggle caption Courtesy of Kassig Family

Smoke rises after a U.S.-led coalition airstrike on Kobani, Syria, Monday, as seen from the Turkish side of the border. Kurdish fighters say they're making progress against ISIS in the area. Tolga Bozoglu/EPA/LANDOV hide caption

itoggle caption Tolga Bozoglu/EPA/LANDOV

A black flag belonging to the so-called Islamic State is seen in the Syrian town of Kobani on Thursday, as seen from the Turkey-Syria border near the southeastern town of Suruc. Umit Bektas/Reuters/Landov hide caption

itoggle caption Umit Bektas/Reuters/Landov

Alleged Islamic State militants stand next to an ISIS flag atop a hill in the Syrian town of Ain al-Arab, called Kobane by the Kurds, as seen from the Turkish-Syrian border in Suruc, Turkey, on Monday. Aris Messinis/AFP/Getty Images hide caption

itoggle caption Aris Messinis/AFP/Getty Images

Smoke rises from the Syrian town of Kobani, seen from near the Mursitpinar border crossing on the Turkish-Syrian border in the southeastern town of Suruc, Sanliurfa province, on Friday. Murad Sezer/Reuters/Landov hide caption

itoggle caption Murad Sezer/Reuters/Landov

Islamic State flags on the Mullah Abdullah bridge in southern Kirkuk earlier this week. A U.N. report says the extremist group has perpetrated possible war crimes and crimes against humanity. Reuters/Landov hide caption

itoggle caption Reuters/Landov

A Kurdish peshmerga soldier who was wounded Sept. 30 in fierce battles in nearby Nineveh province with Islamic State group militants is brought to the Zakho Emergency Hospital in Dahuk, Iraq. Hadi Mizban/The Associated Press hide caption

itoggle caption Hadi Mizban/The Associated Press