Iraq's Prime Minister Haider al-Abadi, speaking Thursday in Washington, said recent battlefield victories showed the Islamic State could be defeated. The extremist group still holds large parts of the west and the north of Iraq. Kevin Wolf/Kevin Wolf hide caption

itoggle caption Kevin Wolf/Kevin Wolf

American journalist Ned Parker (foreground) is the Reuters bureau chief in Baghdad. He fled Iraq last week after receiving threats in response to reports on human rights abuses by Shiite militias allied with Iraq's government. He's shown here at Iraq's Foreign Ministry in 2007. Courtesy of Ned Parker hide caption

itoggle caption Courtesy of Ned Parker

Iraqi Prime Minister Haider al-Abadi and President Obama meet at the White House on Tuesday. The prime minister is visiting to discuss the fight against the self-proclaimed Islamic State. Jacquelyn Martin/AP hide caption

itoggle caption Jacquelyn Martin/AP

Former Blackwater security guards were sentenced Monday for the shooting of dozens of Iraqi civilians in Nisour Square in Baghdad, Iraq. The square is seen here on Sept. 20, 2007, four days after the incident. Khalid Mohammed/AP hide caption

itoggle caption Khalid Mohammed/AP

Members of the Shiite militia group Kataib Hezbollah train at a military base near Samarra, in northern Iraq, on March 5. The Shiite militia stands accused of retaliatory violence against Sunnis in an area it helped retake from ISIS. Baraa Kanaan/EPA/Landov hide caption

itoggle caption Baraa Kanaan/EPA/Landov

Iraqi security forces, most of them Shiite Muslims, gather Thursday in Tikrit at the entrance of a palace that belonged to the former dictator Saddam Hussein. The Islamic State, a Sunni extremist group, had held Tikrit since last summer. When it was in control, the group painted its black flag on a blast wall at the entrance to the compound. Khalid Mohammed/AP hide caption

itoggle caption Khalid Mohammed/AP

Iraqi security forces and allied Shiite militiamen look for Islamic State extremists in Tikrit on Tuesday. Iraqi forces were going house-to-house in search of snipers and booby traps. Khalid Mohammed/AP hide caption

itoggle caption Khalid Mohammed/AP

Kurdish peshmerga fighters keep watch during the battle with Islamic State militants on the outskirts of Mosul on Jan. 21. Azad Lashkari/Reuters/Landov hide caption

itoggle caption Azad Lashkari/Reuters/Landov

David Petraeus speaks in Los Angeles on Nov. 7, 2014. The former head of the U.S.-led coalition in Iraq tells The Washington Post that Iran and Shiite-backed militias pose a greater threat to Iraq than the self-described Islamic State. Imeh Akpanudosen/Getty Images hide caption

itoggle caption Imeh Akpanudosen/Getty Images

Shiite fighters and Sunni fighters, who have joined Shiite militia groups known collectively as Hashid Shaabi ("Popular Mobilization") to fight the Islamic State, gesture Tuesday next to former Iraqi President Saddam Hussein's palaces in the Iraqi town of Ouja, near Tikrit. Thaier Al-Sudani/Reuters/Landov hide caption

itoggle caption Thaier Al-Sudani/Reuters/Landov

Shiite militiamen pose with their banner (right) next to a captured — and upside-down — ISIS flag (left) in Tikrit, Iraq. Militias are bolstering Iraqi forces in a major operation to retake the city from the self-proclaimed Islamic State. Mohammed al-Mozani/AP hide caption

itoggle caption Mohammed al-Mozani/AP

Iraq's push into Tikrit follows its retaking of the nearby town of al-Alam on Tuesday. There, a woman welcomed a relative who is fighting with a militia that's backing Iraqi troops. Thaier al-Sudani/Reuters /Landov hide caption

itoggle caption Thaier al-Sudani/Reuters /Landov

Dura Europos, a Roman walled city in eastern Syria, dates back to 330 B.C. The main gate is shown here in a photo from 2010. It's one of the many important archaeological sites militants of the self-styled Islamic State have ransacked and damaged. EPA /Deir Ezz-Zour Antiquities Department/Landov hide caption

itoggle caption EPA /Deir Ezz-Zour Antiquities Department/Landov

Iraqi security forces and Shiite fighters fire artillery during clashes with ISIS militants in Salahuddin province. The push to retake Tikrit is being aided by Iran, which is providing rockets and other support to Iraq. THAIER AL-SUDANI/Reuters /Landov hide caption

itoggle caption THAIER AL-SUDANI/Reuters /Landov

Members of Iraqi security forces drive toward an area south of Tikrit, Iraq, over the weekend. An Iraqi force has launched a military offensive, hoping to push ISIS out of Tikrit. AHMAD AL-RUBAYE/AFP/Getty Images hide caption

itoggle caption AHMAD AL-RUBAYE/AFP/Getty Images