In 2003, US-led forces invaded Iraq, dismantled the state, and brought an end to Baathist rule.
Chaos followed, giving rise to civil war and laying the foundations of a new order.
Sectarianism is one of the pillars of that order. Until the invasion of Iraq, it was mostly associated with Lebanon, where Christians and Muslims shared power in peacetime, and fought over it during successive civil wars.
But after the invasion, Iraq lapsed into Sunni-Shia civil war, and almost a decade later, sectarianism has been wired into the Iraqi system.
It began with defeat. Abu Muhannad, a former colonel in Saddam Hussein's army who lives in Fallujah, recalls with bitterness the end of his service in 2003, after the army was disbanded.
"A whole segment of Iraqis who served and built Iraq have been forgotten - thrown to the wind overnight," he says.
It was not only army officers. Professors who had no choice but to join the Baath Party lost their jobs in a campaign of "De-Baathification".
"When you're hurt, you don't forget your wound," said Abu Muhannad. "It is a very difficult psychological situation we're in."
As that feeling spread, the Sunni community sank into collective alienation.
But elsewhere, it is a different story.
Nouri Maliki, a veteran of the Shia Islamist Dawa party, is the prime minister of Iraq. Under Saddam, membership of the party was punishable by death.
Religious rituals performed by Shia were also banned under Saddam, who instilled terror into an entire community.
"Clerics would be killed for a book they wrote", said Sayyed Sadr al-Din al-Qubanji, a senior cleric who gives the Friday sermon in Najaf.
No strangers to alienation, the Shia were only just beginning to cast it aside.
In Baghdad, Mr Maliki is now backed by a coalition of Shia movements that have decisive sway within the government.
And in Najaf, a spiritual heartland of Shia, the clerical establishment is on the rise again. Despite a conservative tradition, it is experimenting with power.
"After liberation from the former regime, the clerical leadership saw that it is capable of acting and that circumstances permit political action," said Sayyed Qubanji. "It did not aim to become a political leadership, but to fulfil a role needed by the people."
Meanwhile, there are signs the political leadership is protecting its newfound gains.
Straight after the withdrawal of US troops at the end of last year, a warrant was issued for the arrest of Vice-President Tariq al-Hashemi, the most senior Sunni politician in Iraq.
He was accused of running death squads, tried in absentia, and sentenced to death.
And Mr Maliki has kept both the defence and interior ministries under his control, refusing to hand them over to his partners within the government.
To those who see themselves on the losing side, it is all a vicious attack on their community.
"Off the streets the sectarian war goes on. The dominant side is oppressing the Sunnis," says Abu Ahmed, a spokesman for the Free Iraqi Army.
The militant group was formed to aid the Free Syrian Army, the main armed rebel group fighting to topple Syrian President Bashar al-Assad. In Abu Ahmed's mind, the war extends beyond Iraq.
"Bashar is killing children and women," he says. "This is murder, particularly of the Sunnis. After Bashar it will be Maliki's turn."
Such threats are taken seriously in Baghdad. The government has been tirelessly advertising its panic over events in Syria, and the fear has trickled down to ordinary people.
"It's a Wahhabi war against Shia holy places in Syria," one man in Najaf says. "They also want to cut off supplies to Hezbollah."
"They're targeting the Shia," says another.
Almost every event in Iraq is weaved into two rival narratives.
In a Sunni narrative, Tariq al-Hashemi is the victim of a new Shia dictator, and al-Qaeda is an Iranian tool designed to keep Sunnis locked up on charges of terrorism.
In the rival Shia narrative, Mr Hashemi is guilty as charged, and al-Qaeda is Saudi Arabia's proxy in a regional battle against the Shia.
History is also contested. In one narrative the Baath party's secular Arab nationalism held Iraq together, but in the other it was just a mask for Sunni domination.
But what is real and what is imagined? As sectarianism morphs into a state of mind, the distinction is blurred. Two realities emerge and live side by side, in mutual suspicion.
Fear and alienation
Iraq is no stranger to identity politics. An ethnic divide between Arabs and Kurds has been around since the creation of modern Iraq, and has grown in relevance after the war. But today, identity seems more central to politics than ever before.
In the 1950s, there was a struggle in Najaf between the clerical establishment and a rising communist party.
Young people across Iraq and in Najaf itself were turning to the idea of class struggle as a means to liberation, and the clerical establishment was nervous.
Sayyed Mohammed Baqer al-Sadr, a leading Shia cleric who was executed during Saddam Hussein's rule, wrote a rebuttal of Marxist doctrine and advanced a theory of an Islamic economic system.
Amid the upheaval that followed independence from colonial rule, there was a battle over ideas across the Arab world.
In today's sectarian duality, there's little room for ideas, only a blinding fog of fear and alienation.
"They want to destroy Sunni identity in Iraq," Abu Ahmed warns.
In his world, the Sunnis and Shia are two nations at war.
It is a world in which people see enemies everywhere, and sectarian identity determines how they think.