KARBALA, Iraq — A dozen miles outside this shrine city, on the edges of the uninhabitable western Iraqi desert, a group of paramilitary policemen provides the only visible line of defense against the extremist Islamic State in Iraq and Syria.

Hunkered down in a command post, its walls fortified with concrete and rubble, the policemen’s leader, Col. Hossein Alegeli, is dispatching one of his ranger teams deep into the sultry wastes in search of Islamic extremists determined to destroy the holy shrines of Shiite Islam. The team might be the first line of defense, but as Shiite officials in the area scramble to meet the threat, it is hardly the last.

Most security officials in Iraq’s Shiite holy twin cities, Karbala and Najaf, home to the mausoleums of three of the most revered saints in Shiite Islam, will tell you that the enemy poses little threat because there are many more Shiite men and heavy weapons out there.

Somewhere.

Exactly where, they will not say. Two weeks ago, the spokesman for ISIS, Abu Muhammad al-Adnani, issued a warning to Iraq’s embattled prime minister, Nuri Kamal al-Maliki, saying the extremist group would settle its scores with him and the Shiites, not in Baghdad or in the northern Iraqi shrine city of Samarra, but “in Karbala, the filth-ridden city, and in Najaf, the city of polytheists.”