There were an average 93 attacks each day against Iraqi civilians -- more than three times the rate two years ago.
Overwhelmingly, strife between Sunni and Shiite Iraqis is driving the upsurge in violence. Sunni insurgent groups and Shiite militias carry out kidnappings and bombings in an ongoing cycle of violence and retaliation. In Baghdad, neighborhoods where Sunnis and Shiites once lived together are now being cleared of one group or the other, in what some are calling sectarian cleansing.
"Sectarian violence now presents the greatest immediate threat to Iraq's stability and future," CIA Director Michael Hayden told the Senate Armed Services Committee in November.
Saddam Hussein's Dec. 31 execution illustrated the deep sectarian divide -- Shiites taunted the former dictator in his last moments with chants of the Shiite cleric Muqtada al-Sadr's name.
It wasn't always this way. Sunni and Shiite Muslims in Iraq and elsewhere have a long history of conflict, but also, at many points over the past 14 centuries, of living together peacefully.
The religious divide between the two sects formed over many years, according to University of Michigan historian Juan Cole. It began after the Prophet Muhammad's death in 632, when his followers could not agree on who should succeed him. Some -- the predecessors of the Shiites -- thought Muhammad's son-in-law, Ali, should be the successor, while others -- the Sunnis predecessors -- thought the position should go to a community elder named Abu Bakr.
Abu-Bakr won, and became the first Caliph, or leader of the Muslims. Meanwhile, the predecessors of the Shiites began to call Ali and his descendents "Imams," and considered them their leaders. After the third Caliph was murdered, in 656, Ali did become Caliph -- but then he too was murdered five years later.
When the 11th Imam -- Muhammad's many-times-over great-grandson -- died in 874, a legend grew up among Shiites that his young son, the 12th Imam, had disappeared from the funeral. Many began to see the child as a messianic figure, furthering distancing the Sunni and Shiite forms of Islam.
"By the time you get to the 900s, you can really talk about Shiism as a separate movement," Cole explained. "From the Shiite point of view, history went wrong. The central rituals in Shiism are about mourning, in Sunnism there's no equivalent."
But, Cole and others have said, this religious aspect of the Sunni-Shiite split has little to do with the contemporary violence in Iraq.
"It's a struggle for political power," said Vali Nasr, a professor of national security affairs at the Naval Postgraduate School in Monterey, Calif. "Whenever you have identity divisions, there's always an identity marker. Sometimes it's race, sometimes language. But sometimes religious identity becomes the marker, and it has nothing to do with whether or not you're pious."
In Saddam Hussein's Iraq, the divide between Sunnis and Shiites simmered although it rarely erupted, according to Nasr. Saddam, a Sunni, filled the top ranks of his Baath Party with other Sunnis. So although Iraq was and remains about 60 percent Shiite, Sunnis were the ruling class.
"Some people say there was no sectarianism, but that's like saying there was no racism in South Africa under apartheid," Nasr said. "The state was racist. There was no sectarianism in the streets, but that doesn't mean there was no sectarianism."
Still, the Baath Party was a secular party, and on a day-to-day level Shiites and Sunnis could live together in mixed neighborhoods, work together and marry each other --particularly in cosmopolitan Baghdad.
"By and large the people were moderates, and the religious element was a small element in their lives," said Laith Kubba, the Iraqi-born senior program officer for the Middle East and North Africa at the National Endowment for Democracy, and a former spokesman for Iraqi Prime Minister Ibrahim al-Jaafari. "You'd find more extremist Shiites in Pakistan and Iran, and more extremist Sunnis in Saudi Arabia."
What has occurred since the fall of Saddam to incite the violence is a power vacuum, said Kubba.
"You have 20 million-plus people, with no services to provide law and order. And this in a country that was so dependent on bureaucracy," he said. "When you pull out state and government, the only thing left is the mosque."
Insurgent groups -- many made up of Sunnis deposed from power -- began attacks against Americans and Iraqis almost immediately after Saddam's fall. Then, particularly after the February 2006 bombing of the al-Askari mosque in Samarra -- one of the holiest sites in Shia Islam -- retaliatory violence by Shiite militias began in earnest.
In the past several months, the violence has only gotten worse. A December article in the Washington Post, for example, told the emblematic story of a Sunni family about to flee their home in Tobji, a previously mixed neighborhood in central Baghdad where they had lived since the 1950s, as Shiite militiamen kidnapped and killed other Sunnis in the area.
"I am scared. I am Sunni," the father, Farouk, told the paper.
Meanwhile, many moderate, secular Iraqis have fled the country. "The people who intermarried were the first to leave," Nasr said. "Most of the class in which intermarriages happened--the middle class--they've left Iraq."
In politics, the parties that have replaced Saddam's Baath Party are all sectarian. The Iraqi parliament includes the Shiite Supreme Council for Islamic Revolution, the Shiite Sadr movement, and the Sunni Iraqi Islamic Party, among others.
Many of these political parties also have armed wings driving the violence on the ground, including Muqtada al-Sadr's Mahdi Army and the Supreme Council for Islamic Revolution's Badr organization.
The Iraqi government has made some attempts to bridge the sectarian divide. In mid-December, Prime Minister Nouri al-Maliki organized a "National Reconciliation Conference" in Baghdad, intended to bring together Sunnis, Shiites and Kurds to discuss how to end the violence. At the conference, al-Maliki invited former members of Saddam's Baath party to join Iraq's new army.
But most accounts deemed the conference a failure, particularly because many key groups, including the Shiite Mahdi army and the Sunni Muslim Scholars Association, refused to attend.
Meanwhile, the relationships among all these political and insurgent groups are complex. According to Cole, for example, Muqtada al-Sadr paradoxically has a better political relationship with some Sunni members of parliament than any other Shiite, despite the fact that his Mahdi Army and Sunni insurgents are at war in the streets every day.
That's because, Cole wrote in his blog, al-Sadr sees a difference between most Sunnis and those Sunnis who violently hate Shiites, and he believes his militia is fighting only the latter group.
But, according to Cole, a stable coalition between Sadr and Sunnis is unlikely. "I fear that ... if the U.S. left, the Sadrists and the Sunni fundamentalists would gradually fall on one another. Dislike of the U.S. presence is after all among the main things they have in common, and that would be gone," he wrote.
And that sort of stable coalition is the only hope for reducing sectarian violence, according to Kubba.
"Put the state back to business and you'll immediately pull back a huge chunk of Iraqis," he said, "and cut the religious zealots back to size."