Political violence in Iraq has become a fact of life, the threat of imminent death a practical consideration for any Iraqi who chooses to attend a crowded market, travel by bus or car between towns or neighborhoods of major cities, commemorate religious holidays, or publicly mourn for friends and relatives of past attacks.

Yesterday was no different. A suicide bomber targeted a funeral tent in the Baghdad neighborhood of Doura, a few miles south of what was once known as the Green Zone and that today houses a sprawling, underused US embassy built at a cost of $750 million. The attack, in a Sunni-dominated neighborhood that was among the city's bloodiest at the height of the civil war, is a reminder that death stalks Iraqis even in the heart of the capital. 

Elsewhere on Thursday two suicide bombers targeted Shiite pilgrims in the crossroads town of Latifiyah, south of Baghdad, killing 20. The pilgrims were making their way to the ancient shrine city of Karbala to commemorate the death of Hussein, a grandson of Islam's prophet Mohammed, and were passing through a region that has long been referred to as the "triangle of death."

Although political violence has been a fact of life for Iraqis since the US-led invasion that toppled Saddam Hussein in 2003, it had declined substantially since 2008. But the death rate for civilians more than doubled this year, raising the troubling prospect of Iraq possibly slipping once more into outright chaos.

How bad is it? This year has been by far the deadliest for civilians since at least 2008, with well over 7,000 killed in roadside bombings and attacks on markets, homes, and mosques.

When the US and other foreign militaries left the country at the end of 2011, there were warnings that violence could spike again. That didn't happen immediately, as United Nation's data shows, but Iraq is once more one of the two or three hottest wars in the world.

These are the totals of civilians killed in Iraq since 2008, based on data compiled by the United Nations Assistance Mission to Iraq:

The 2013 total doesn't include killings in December – and doesn't include Iraqi soldiers and militants who have died. Data compiled by Agence France-Presse – whose methodology differs from the UN's – had at least 337 Iraqi civilians killed so far this month, and that was before the 36 people murdered yesterday.

Why has 2013 been so bad? Was it because the US withdrew combat troops at the end of 2011? (US hopes of an extended presence fell flat when the government of Prime Minister Nouri al-Maliki declined to grant immunity to US troops from Iraqi prosecution.) Well, the increase in violence in 2012 was modest, certainly compared to the change this year. The absence of large numbers of foreign troops has certainly played a role in creating more space for insurgents and militias.

But a key factor seems to be the explosion of the civil war in Syria, which breathed new life into Al Qaeda in Iraq and its fellow travelers. Iraqi fighters have flowed into Syria, many from Anbar province that was a stronghold for Sunni insurgents during the height of the US war, just as Sunni fighters flowed from Syria into Iraq in the middle of the last decade. The Sunni militant group the Islamic State in Iraq, which once branded itself Al Qaeda in Iraq, renamed itself to the Islamic State of Iraq and the Levant this year – explicitly acknowledging its connection to the Syrian war.

Violence remains well below the levels that preceded the so-called US troop surge in Iraq that began in early 2007 and ran through the middle of the following year. But the success of the surge largely hinged on encouraging Sunni Arab insurgents to switch sides, convincing them to link up with US and Iraqi government forces in exchange for the promise of jobs and inclusion in the country's emerging political institutions.

But in the years since, Iraq's dominant Shiite politicians have continued to push Sunnis aside and the promised jobs evaporated, a process that accelerated with the US departure from the country at the end of 2011. Political reconciliation never took place, and an angry, disenfranchised Sunni community is fertile ground for recruitment of fighters.

In some ways, the violence just burned itself out. Sectarian cleansing took place across Baghdad and other cities, turning what had once been mixed Sunni-Shiite neighborhoods into strongholds for one side or the other. In short, death squads had fewer easy targets and accomplished a short-term objective. But that adds to concerns over the 2013 surge in violence: Even with this sectarian sorting, the "new" Iraq is a bloody place, where politics are driven by sectarian allegiances, and where their seems to be no shortage of men willing to kill civilians.

As the US continues to debate what it should do about Syria, where the civil war has internally displaced more than six million people and turned two million into refugees abroad, Iraq is a reminder that even massive spending and a sprawling occupation are no guarantees of great success. The US spent more than $800 billion on the war in Iraq and counted on a long-term involvement with the country that would strengthen the interests of both.

The US Embassy in Baghdad, the largest in the world, is a symbol of how things actually turned out. When the embassy was opened to great fanfare in 2009 – then ambassador to Iraq Ryan Crocker said at the time "as our military presence ramps down, many other aspects of our relationship are going to ramp up"– the expectation was that an army of US diplomats and contractors working on aid and development projects would be in the country indefinitely. At that time the embassy was responsible for 16,000 employees across the country. By the end of this year, it's expected to be responsible for fewer than 6,000.