Iraq is disintegrating. The astonishing gains made by ISIS, an al-Qaeda-linked jihadist group that has grown in strength amid the chaos of Syria's civil war, now sees extremist Islamist militias threatening even the capital of Baghdad. On Tuesday, ISIS fighters overran Mosul, Iraq's second largest city, which could become a strategic linchpin in the group's quest to carve its own Islamic state out of modern day Syria and Iraq. Hundreds of thousands have been displaced by the violence in the space of days.

As ISIS fighters occupy a host of cities, other factions are also entering the fray. Kurdish militias seized the long-contested oil-city of Kirkuk on Thursday, while Iran, with close ties to the pro-Shiite government in Baghdad, sent in two battalions of its crack Revolutionary Guards to help beat the jihadists back. The conflict has dangerously exposed once more Iraq's troubled sectarian divides. Here’s WorldViews’s primer on some of the key characters in the battle for Iraq.

ISIS: The jihadist group, according to my colleague Liz Sly, 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.

Reports on Wednesday suggested that ISIS's ranks may have grown after collaboration with militias connected to the old Baathist regime of fallen dictator Saddam Hussein; they played a role in ISIS's reported capture of Tikrit, Hussein's home town. ISIS appears to be well-funded, benefiting from the same shadow networks of donors in the Arab world who funded al-Qaeda as well as the widespread practice of extortion, kidnapping  and other criminal activities.

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. The divisive rule of the Shiite-dominated government in Baghdad has, in part, been blamed for the hopeless security situation in the country's Sunni-majority areas. The units that retreated, shorn of uniforms and much of their armaments, will have to reassemble with reinforcements from Baghdad, but that may be difficult, given the number of battles the government is already fighting with ISIS around towns nearer to the capital. Iraqi helicopters and fighter jets have struck ISIS positions across the country, but the militants are still reportedly advancing closer to Baghdad.

Kurds: The autonomous government in the Iraqi region of Kurdistan has rallied its own forces, known as the pesh merga, 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. The pesh merga have long eyed Mosul, which has a significant Kurdish population and lucrative oil fields. They possess some light armored vehicles as well as artillery and will probably be the key to winning back Mosul. That, as the Checkpoint blog notes, could in the long run lead to renewed Kurdish claims and further tensions with Baghdad. Kurdish fighters raised their flag over Kirkuk on Thursday, a grim sign of Iraq's gradual partitioning.

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, the security situation in predominantly Sunni areas has unraveled to the point that vast stretches of the country have slipped out of Baghdad's hands.

President Jalal Talabani: The venerable Kurdish politician played a key role in establishing an autonomous Kurdish region in northern Iraq. Since 2005, he has served as Iraq's president, and has had a testy relationship with Maliki.

Tariq al-Hashemi: Just a day after the official U.S. withdrawal in 2011, Maliki's government issued an arrest warrant for the then-Vice President Hashemi, the top  Sunni politician at the time in the country. Hashemi, accused of crimes related to terrorism, went into hiding. The government plunged into crisis and Iraq's Sunnis rose up in mass protests against Maliki. Hashemi was later sentenced to death in absentia and remains a fugitive.

 Abu Bakr al-Baghdadi: The Iraqi-born leader of ISIS, who has a $10 million U.S. bounty on his head, has been hailed 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.

Moqtada 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's 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 reportedly 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. Renewed protests and unrest in Kurdish towns in Turkey make the situation all the more delicate.

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 on Wednesday promised Baghdad support in its fight against "terrorism." On Thursday, reports emerged that two battalions of the Quds Force, the overseas wing of Iran's Revolutionary Guards, were in Iraq, helping the government battle ISIS. According to the Wall Street Journal, a combined Iraqi and Iranian force reclaimed parts of the city of Tikrit.

Army of the Men of the Naqshbandi Order: The Naqshbandiya take their name from a Sufi Muslim order, but in reality there's nothing mystical about them. The Sunni jihadist militia counts a number of fighters who were once affiliated with the Baathist regime of toppled dictator Saddam Hussein. They have reportedly collaborated with ISIS during its recent march toward Bagdhad.

Asaib ahl al-Haq: A Shiite militia that was active during Iraq's hideous sectarian warring in 2006 and 2007, it is mobilizing once more, alongside other similar factions, to counter the threat posed by ISIS.