Key characters in the battle for Iraq

ISIS: The jihadist group the Islamic State of Iraq and Syria has a fighting force that is probably larger than the 10,000 or so members estimated in most reports. They are well-armed and have boosted their arsenal after looting equipment from Mosul's main army bases. In every city they overrun, ISIS frees hundreds of prison inmates, some of whom may be like-minded militants.

Iraqi army: The Iraqi army in Mosul wilted in the face of the ISIS assault. Despite billions of dollars spent by the United States in training the post-Hussein army, it suffers from poor organization and morale. Two Iraqi divisions — an estimated 30,000 troops — stationed near Mosul reportedly ran from an initial ISIS offensive that may have numbered just 800 men.

Kurds: The autonomous government in the Iraqi region of Kurdistan has rallied its own forces to combat ISIS. Although the Kurds have had an adversarial relationship with Baghdad for quite some time, reports suggest that they are now more closely coordinating efforts to counter ISIS.

Prime Minister Nouri al-Maliki: Iraq's premier rose to prominence in the wake of the U.S. invasion and, as a leader of a prominent Shiite political party, assembled the government in 2006. He has managed to remain in power since, but presides over a deeply polarized political landscape. Critics accuse Maliki's government of marginalizing the country's Sunnis while strengthening his Shiite political base. As a result, vast stretches of the country have slipped out of Baghdad's hands.

Abu Bakr al-Baghdadi: The Iraqi-born leader of ISIS, who has a $10 million U.S. bounty on his head, has been described as "the new bin Laden" by Time magazine and, while a deadly ideologue, has managed to assemble an incredibly cohesive, capable outfit that is well-funded and well-armed.

Muqtada al-Sadr: The fiery Shiite cleric, once a thorn in the side of the U.S.'s occupying forces, announced a plan to muster "Peace Brigades," or Shiite militia charged with protecting communities and shrines vulnerable to ISIS' advance. This gives the conflict an even more dangerous and sectarian edge, something the Sunni extremists in ISIS probably hoped to achieve.

Turkey: ISIS has captured several Turkish diplomats, as well as other Turks, in Mosul. It raises fears of a growing regional conflagration. Turkey is in an awkward position: ISIS fighters in Syria have routinely wound up in hospitals in Turkish border towns, rumored to be tolerated by Ankara because of their own battles with Kurdish militias in Syria.

Iran: The Shiite state will look upon developments in Iraq with great concern. ISIS is a real foe, and its success in Syria and Iraq is an existential challenge to two staunch allies of Tehran. Iran's foreign minister promised Baghdad support in its fight against "terrorism."

Grand Ayatollah Ali al-Sistani: Iraq's top Shiite cleric on Friday exhorted all able-bodied Iraqis to take up arms to combat Sunni extremist militants.

Washington Post