Iraqi Shiites of the Mahdi Army militia loyal to cleric Moqtada al-Sadr vowed to fight ISIS in a show of strength in a military parade in the Sadr City district of Baghdad on June 21, 2014. Sadr's militia fought US and Iraqi troops during the last war there, and also participated in sectarian attacks on Sunni Arabs. Scott Peterson/Getty Images/TCSM
The proliferation of militias loyal to political movements and individuals has long been a major problem in Iraq, where the government has far from a monopoly on the use of force. These Mahdi Army fighters say they want to fight Baghdad's enemies now, but could one day turn their guns on the government. Scott Peterson/Getty Images/TCSM
A family poses beside a Shiite religious banner at the Mahdi Army parade. Banners extolling the virtue and bravery of Mahdi Army "martyrs," and seeking to connect the movement to revered Shiite Imams Hussein and Ali, are common in Shiite areas. Scott Peterson/Getty Images/TCSM
This Mahdi Army militiaman in a skull face mask is a reminder that Iraq's militias don't always exercise what would be considered good military discipline. Mahdi Army members participated in the assassination and intimidation campaign in the middle of the last decade that saw many Sunni Arab families driven from their neighborhoods in Baghdad. Scott Peterson/Getty Images/TCSM
The man in the middle of this poster on a Mahdi Army car is Grand Ayatollah Mohammed Sadeq al-Sadr, Muqtada al-Sadr's father. The man to his right is Ayatollah Mohammed Baqir al-Sadr, Muqtada's uncle. Both men were killed by the Saddam Hussein regime for their religious and political activism - something that adds fuel to the Mahdi Army's desire to prevent a return of Sunni Arab political supremacy to Iraq. Scott Peterson/Getty Images/TCSM
Another Mahdi Army vehicle in Sadr City. Scott Peterson/Getty Images/TCSM
Iraqi Shiite tribesmen parading in support of the Mahdi Army. In the late 18th and early 19th century, many of the tribes of Iraq's south converted to Shia Islam, which remade Iraq's religious balance. The consequences of that shift are still being worked through today. Scott Peterson/Getty Images/TCSM
The Sadr movement has close ties to Iran - but is also staunchly nationalist in its outlook. Many of Iraq's Shiites fought and died in the Iran-Iraq war in the 1980s and Iraqi flags are seen as often at the Mahdi Army's rallies as religious banners. Scott Peterson/Getty Images/TCSM
Iraqi Shiites of the Mahdi Army militia loyal to cleric Moqtada al-Sadr lean on rocket launchers as they vow to fight ISIS in a show of strength in a military parade in the Sadr City district of Baghdad, Scott Peterson/Getty Images/TCSM
The talk doesn't mean the US is on the verge of extending its air campaign against Islamic State fighters into Syria, or shifting from an adversarial to cooperative stance with Syrian leader Assad in the interest of defeating IS.
President Obama says the United States will be “relentless” in going after the Islamic State, the militant organization that brutally executed American James Foley this week and whose advances in northern Iraq from its base in Syria prompted the president to order airstrikes earlier this month.