The Shia in my observation* are obsessed with suffering and, as might be said in America, immerse themselves in a culture of being the victim. During Ashura they beat and cut themselves to show their devotion to Husayn and his suffering. They also have a bizarre tradition where they put their sons in a tent which is then lit on fire. All the kids are supposed to run out, but we have had to treat some poor boys who were badly burned. (As an aside, we have treated a lot of burned children and women, apparently there is something wrong with Iraqi stoves in that hot oil is always falling off them) Anyway, the relevance here is to show their devotion to Husayn and the suffering of his family many Shia walk to his shrine in Karbala. They walk from nearby Babil, and Baghdad, but also from Najaf, Basra, even I was told Kuwait and Iran. That is a serious walk, and the sight of hundreds and hundreds of these Shia, all dressed in black, walking down one whole side of the highway, which was closed off for the holiday, is indescribable. Mile after mile off to the horizon of black ninjas. Ah and the noise and the blaring mosque speakers all day long.
As you might imagine, this is a nightmare from a security perspective. It would be easy for a suicide bomber to blend in with crowd. Imagine having to shut a major metropolitan area down to vehicle traffic and protect marchers walking in from all the major highways. An impossible feat, but remarkably, the Iraqis mostly pulled this off, there were several attacks in Baghdad and one suicide bomber in northern Babil (where the Sunni/Shia line is), but it could have been much worse. And I have heard of no attacks in Karbala. We saw the Iraqi Police, Army, and paramilitary forces out on the road everyday, all day, tirelessly working to keep these marchers safe. There were even full blown security plans, operations orders, etc all prepared by the Iraqis by themselves. It was also the best example of Iraqi Army - Iraqi Police cooperation seen recently, and offers hope that these two organizations who distrust each other, but on whom the hope for this country rests, will work out their issues for the benefit of the Iraqi Nation.
*Disclaimer: I am no expert on Shia Islam and can only relate what I have been told by my Muslim friends and Iraqi colleagues.