Karbala and Najaf, two central Iraqi cities that coalition forces reached by Monday, are the holiest sites of Shiite Islam and important to all Muslims.

If battles do break out at either location, allied commanders will likely take precautions to avoid damaging any holy structures or sites.

Najaf is the burial place of Ali, the nephew and son-in-law of the prophet Mohammed, and the fourth Muslim ruler, or Caliph, after Mohammed's death.

Ali's assassination in 661 A.D. marked the breach between Sunni Muslims, who accept as legitimate the Ummayad and Abbasid dynasties of caliphs who followed, and Shiite Muslims, who regard only the prophet's descendants as the legitimate leaders of Islam.

After the death of the first Ummayad caliph, Muawiya, a succession struggle broke out between his son Yazid and Hussein, son of Ali and grandson of Mohammed.

In 680 A.D., Hussein's army was ambushed at Karbala, where both he and his younger brother Abbas were killed and many members of their families executed. The eldest brother, Hassan, had been poisoned earlier.

The martyrdom of Hussein is extremely important to Shiites, akin to the crucifixion of Jesus for Christians. The festival of Muharram is celebrated with mourning, self-flagellation and re-enactments of the battle of Karbala similar to the Passion Plays of Catholic Europe.

Shiites make up an estimated 60 percent of the population in Iraq, according to estimates. But they have been politically marginalized by Saddam’s government, which is controlled by Sunni Muslims.

The Shiites of southern Iraq, where they far outnumber Sunnis, have periodically rebelled against the Baghdad regime. The most serious such rebellion came during the Gulf War in 1991, when the Shiites took up arms in anticipation of support from the United States.

The rebellion was brutally crushed by the Iraqi military when the United States failed to support the uprising.