Extremist Sunni forces are racing toward Baghdad, raising fears that a civil war could engulf Shiite-ruled Iraq and driving hundreds of thousands of Iraqis from their homes.
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Here's a look at some of the key players in the current crisis:
Abu Bakr al-Baghdadi
Baghdadi, whose real name is believed to be Awwad Ibrahim Ali al-Badri al-Samarrai and who claims to be a direct descendant of the Prophet Muhammad, is the leader of the Sunni Muslim militant group Islamic State of Iraq and the Levant, or ISIS.
ISIS was initially part of al-Qaeda in Iraq, the umbrella group of militant Islamists that emerged after the U.S. war in Iraq. Baghdadi succeeded Abu Musab al-Zarqawi as leader of what was then called the Islamic State of Iraq when a U.S. jet killed Zarqawi in 2006.
Under his leadership, the group declared a goal of establishing a single Muslim state across the Persian Gulf region based on Shariah law, employing methods so extreme that it was denounced even by core al-Qaeda leaders themselves. The group added "and the Levant" to its name last year to formalize its wider ambitions.
The U.S. is offering a $10 million reward for the capture of Baghdadi — whom it released from its own prison camp in Bucca in 2009 after more than three years in U.S. custody.
Nouri al-Maliki
Maliki, 63, has been prime minister of Iraq since 2006, having been a spokesman for the coalition of Shiite parties that won Iraq's 2005 elections. He was a leading Shiite opponent of the late Saddam Hussein and lived for a quarter-century in exile after he was sentenced to death in the late 1970s.
Maliki first OK'd the 2007 U.S. "surge" in Iraq and then hailed the withdrawal of U.S. forces as a "new dawn" four years later.
President George W. Bush called Maliki a "good guy, a good man" in 2007, but his dealings with Washington have been tense since President Barack Obama took office in January 2009. Obama's first secretary of state was Hillary Rodham Clinton, who had publicly called for Maliki's ouster during the 2008 presidential campaign.
Maliki has come under criticism for a perceived lack of willingness to crack down on Shiite militias and his occasionally friendly remarks about Shiite Iran.
Tariq al-Hashemi
Hashemi, 71, was the highest-ranking Sunni official in Iraq as vice president until he fled the country in 2011 after he was charged with running political death squads targeting Shiite officials. He was convicted in absentia in September 2012 and sentenced to death.
The conviction of Hashemi, who is reportedly living in Turkey, was one of the most explosive flashpoints sparking Sunni unrest and militancy in recent years. He called for Maliki's ouster in February, and earlier this week he endorsed the ISIS offensive, calling it a "revolution of the oppressed."
Muqtada al-Sadr
Sadr, 40, holds no office, but the charismatic Shiite cleric has long been one of the country's dominant figures.
As head of the feared Mahdi Army, Sadr was a leading military opponent of the U.S. invasion and occupation of Iraq and was a frequent target of U.S. raids and capture missions.
Sadr mysteriously withdrew from public affairs in February, dissolving his political party and announcing that he was unaffiliated with any faction. His only explanation was the puzzling statement that he was seeking to protect his family's name.
But he re-emerged this week by announcing he was forming a new brigade to defend Shiite holy sites — deeply antagonizing ISIS and adding an even deeper sectarian tint to the country's violence.
Grand Ayatollah Ali al-Husseini al-Sistani
Sistani, 80, is the reclusive top Shiite cleric in Iraq and one of the icons of the resistance to Saddam Hussein before the 2003 U.S. invasion.
Proclamations by Sistani in 2003 and again in late 2004 are widely credited with having given Iraqi Shiites justification for supporting the U.S.-backed constitution and elections in 2005.
For years, Sistani preached that his followers shouldn't respond with violence to attacks from Sunni militants — not even in response to the destruction of the revered al-Askari mosque in 2006, which led to a civil war that killed thousands of people.
But this week, Sistani issued a stunning statement read at Friday prayers calling on Shiites to take up arms and "volunteer to join the security forces" fighting the ISIS insurgency.
Saddam Hussein
Hussein, of course, has been dead since he was executed in December 2006. But he and his 24 years of Sunni Baath Party are at the heart of everything that has happened in the modern history of Iraq.
The militant groups that gave rise to al-Qaeda in Iraq, ISIS and other anti-Shiite, anti-U.S. organizations spawned from the ashes of Hussein's Baath Party military and paramilitary.
Top former commanders who served under Hussein are among the leading strategists and commanders of the ISIS insurgency, among them Abu Ayman al-Iraqi, a top intelligence colonel in Hussein's army.
A triumphalist audio recording emerged this week purportedly from ISIS. Its occasion? The ISIS capture on Wednesday of Tikrit — Hussein's hometown.
Polly DeFrank and Helen Kwong of NBC News contributed to this report.
