They walk through the streets drenched in blood, hitting their heads with knives as a drummer keeps up a steady beat. They chant the name of Imam Hussein, their white garments stained red with all they have spilt in his name.  This is Karbala, 2013. While most here in the crowd dress in black and tap their chests to show their reverence, this fervent group of young men replay a violent tradition that recalls a battle fought 1,300 years ago.  And when they recreate the battle there are no half measures. A horseman leads a crowd of thousands through the streets, a run from the scene of the battle some 4km away to the shrine. It's an extraordinary sight. From the air it looks like the whole city is moving. Red and green flags wave proudly and the mass crowd chant ever louder.  The re-enactment is the culimation of Ashoura, the commeration of the death of Imam Hussein in the Battle of Karbala, 680 CE. He was the grandson of the Prophet Muhammad.  The Iraqi authorities say more than four million people have arrived in the city. Everywhere you look is a sea of humanity. Dotted throughout the crowd I see some with tears streaming down their faces. I stop one of them to ask why such a show of emotion. He's a young man, 16 years old from London in the UK. This is Irfan's first time in Karbala.  "Imam Hussein showed us the true way of Islam. Our lives are owed to his sacrifice. After hearing the stories of how he died in battle I can't believe I'm here, I can't believe his shrine is in front of me".  Irfan's words are common here. The faithful have come from all over the world, with little fear of the violence that has plagued Iraq they come out of religious duty, love and curiosity.  For the Iraqi authorities, the safety of Irfan and the millions of others is top priority. The army has mounted its biggest operation in 10 years to secure Karbala.  It begins with a ring of checkpoints 15km outside of the city. The faithful leave thier vehicles behind and walk in. Every few miles there are checkpoints and imposing blast walls, with the camouflage of the army uniforms contrasting with the dull black of the mourners.  Some 35,000 troops are involved. Prime Minister Nouri Al-Maliki is said to be taking a close interest in Karbala and is determined to make it pass without incident.  But in the last 24 hours suicide bombers have struck smaller cities in countryside targeting Shia processions. At least 14 have been killed and dozens more injured. This is now the state of the conflict in Iraq, a war that is more intense now than at any time since height of sectarian violence in 2008.  Many here pray that the Prophet granted Imam Hussein the highest place in Paradise.  But many others pray that the loved ones they have lost in sectarian violence have joined him.
The Shia ceremony marking Imam Hussein's death 1,333 years ago is also a time to remember those lost in recent battles.