All week, Shi'ites from across Iraq and neighboring states make a pilgrimage on foot to Kerbala.<br/> <br />It's for a religious ceremony called Arbain that culminates Saturday and marks the death of Imam Hussein 13 centuries ago.<br/> <br />They'll come by the millions, many walking for days.<br/> <br />Hundreds of thousands have already come from Iran alone.<br/> <br />But security is tight, as authorities fear Sunni fighters from Islamic State plan to "infiltrate" crowds and launch attacks against pilgrims.<br/> <br />(SOUNDBITE) (Arabic) SHI'ITE PILGRIM FROM BASRA, FADI ABBAS, SAYING:<br/> <br />"People from all provinces and all countries are flocking to the city. Thank God, this visit is seen as a challenge to Islamic State and all those who support and back them."<br/> <br />Security forces face a double challenge.<br/> <br />Many troops are diverted to tackle Islamic State fighters elsewhere in the country.<br/> <br />It's forced the government to rely more heavily on Shii'te militias to keep order.<br/> <br />But the governme
