This post was updated at 9:40 p.m. ET to reflect the Obama administration's pressure on the Iraqi government.
A week ago, it would have been difficult to find anyone in the U.S. arguing for renewed U.S. military action in Iraq. Now there's a furious debate about what the U.S. should, or shouldn't, do in the latest Iraqi crisis.
The drama seemed to erupt out of nowhere as Islamist extremists captured Mosul, one of the country's largest and most important cities, and kept pushing south toward the capital Baghdad.
In reality, this crisis was years in the making in a country where the three main groups, the Sunnis, the Shiites and the Kurds, have never really made peace with each other.
So what's likely to happen? Here are some of the key things to watch. We'd also like to hear from you. Send us your questions about Iraq in the comments section, and we'll do our best to answer them.
1. Can the Sunni militant group ISIS march all the way to Baghdad and take the capital?
ISIS, the Islamic State of Iraq and Syria, is still a relatively small force of fewer than 10,000 fighters compared with an Iraqi army of some 250,000, plus a large police force. So far ISIS has rolled through mostly Sunni areas where they met little resistance. That's already begun to change as the militants reach mixed Sunni-Shiite towns. And as they get closer to Baghdad, they will hit mostly Shiite areas and can expect much tougher going.
"It would be very difficult for ISIS to take Baghdad," Lina Khatib, director of the Carnegie Middle East Center, told NPR. "The [Iraqi] army in Baghdad is more tightly linked to the government itself. And also, physically, only a few hundred ISIS fighters actually took over Mosul. It would be difficult for them to replicate this on a larger scale in Baghdad."
However, ISIS is receiving help from other Sunni militias along the way, and ISIS has seized money and weapons as they take over cities and Iraqi army bases. The group has shown no signs of slowing down and attacked the country's largest oil refinery in Beiji on Wednesday.
The group could certainly wreak havoc in Baghdad with suicide bombings and other attacks, but taking control of the city would be another matter.
ISIS may want to consolidate its hold on Sunni areas rather than launch a full-scale confrontation with Shiites on their home turf.
One possible military scenario is an Iraq that is effectively divided into three separate regions along sectarian lines. ISIS would be dominant in the Sunni areas of the west and north, the Kurds would hold sway in the northeast, and Prime Minister Nouri al-Maliki and the Shiites would control Baghdad and the south.
But Feisal Istrabadi, Iraq's former U.N. envoy, warns that "if Iraq falls apart, it will not fall apart into three neat pieces. What you're much more likely to see is a Somalia in Iraq."
2. Is Iraq's government in danger of collapsing?
Maliki is a political survivor who has ruled Iraq for eight turbulent years. His party just won the most seats in elections held in April, though the new government hasn't yet been established. And the Iraqi army, despite its disastrous showing in recent days, is still considered the largest and most powerful force in the country. So while Maliki's grip looks shaky, he does not yet appear to be in imminent danger, according to most analysts.
However, he's now running a much smaller patch of Iraq that's largely limited to his Shiite base. The U.S. has repeatedly called for Maliki to reach out to the Sunnis, yet the the Iraqi leader consistently alienated them. In one of many examples, Maliki removed many Sunni generals from the army after the U.S. military withdrew from Iraq at the end of 2011.
Leslie Gelb, president emeritus of the Council on Foreign Relations, argues that Iraq should look to its constitution, which allows for a federal system that would let all three groups run their own areas.
"I don't know whether it will work today, but I think that's the only chance," Gelb told NPR. "Even though it's a long shot, it's the only chance of keeping Iraq together and providing some incentive for Sunnis to stay away from the jihadis and remain a part of the Iraqi state."
Update at 9:40 p.m. ET: A senior official tells NPR that the U.S. is pressuring the Iraqi government to include the Sunni minority, but doesn't believe the prime minister can achieve that. The official said if Maliki can't make that change, Iraq may need new leadership.
3. Is the U.S. likely to take military action?
None of President Obama's options are appealing. He clearly has no enthusiasm for renewed military action in Iraq, and neither the president nor anyone in his administration has defined what the military objective would be.
Yet it would be a major blow to Obama if Iraq's elected government was violently overthrown. In addition, the U.S. has a massive embassy in Baghdad, and a complete evacuation would be humiliating, evoking comparisons to the end of the Vietnam War in 1975.
The president has ruled out U.S. troops on the ground, which means a response would focus on air power. But the vast U.S. intelligence network in Iraq has been dismantled, and even airstrikes require precise, real-time information from the ground.
Iraq is requesting help. Iraq's ambassador to the U.S., Lukman Faily, told NPR:
"We are, we have, we will, do this fight. We need support and training. We need support and logistics. We need support and air supremacy. We definitely do not need two combat forces. We definitely do not need boots on the ground from the United States. So that's the level of the support we have requested."
The Obama administration is considering highly selective airstrikes rather than a widespread bombing campaign, The New York Times reported Wednesday.
Such a campaign could resemble the U.S. drone strikes in Yemen, Pakistan, Somalia and Afghanistan. While the individual attacks are often successful, the air campaigns have not decisively resolved any of those conflicts.
4. How is the crisis playing out in the region?
The fighting in Syria and Iraq is looking more and more like a single, massive conflict, with ISIS operating freely on both sides of the border. There's a consensus among Western and Middle Eastern states that ISIS is a major threat, yet no foreign government has so far proved willing to step in and directly confront the group.
The main U.S. worry is that large parts of Iraq and Syria could become ungoverned territory that serves as fertile ground for jihadists with designs on the U.S.
The Syrian war has already created a massive humanitarian crisis, and a new one looks to be rapidly developing in Iraq.
Oil markets are nervously watching to see what happens to production in Iraq.
Iran, which has few friends, is already propping up Syria's President Bashar Assad, a member of the Alawites, an offshoot of Shiite Islam. Now Iran faces the prospect of sending substantial military assistance to Maliki, a fellow Shiite in Iraq.
Nothing better illustrates the alarm over Iraq than the sudden willingness of the U.S. and Iran to discuss ways of stabilizing Iraq. Those talks were held this week in Vienna on the sidelines of the nuclear negotiations on Iran's nuclear program.
"Iran and the U.S. have a mutual interest in crushing al-Qaida movements in the region because they both view it as a security threat," Farnaz Fassihi, of the Wall Street Journal, told NPR. "So there is a common interest to make sure that the Iraqi government is able to survive and that stability is returned to Iraq."
Greg Myre, the international editor of NPR.org, spent a decade reporting from the Middle East. Follow him @gregmyre1.