The holy cities of Najaf and Karbala, located deep in the Shia heartland of southern Iraq, traditionally opposed the secularist Baathist rulers in Baghdad.
Karbala is an important place of pilgrimage for Shia Muslims
They are home to some of the most magnificent and sacred shrines of the Shia branch of Islam and are leading centres for scholars of Islamic theology.
The fabled shrines and colleges were for a long time under the rule of the Baath Party. Saddam Hussein, who claimed descent from the Prophet, put his own pictures on the shrines.
And many Shia clerics from the holy cities were murdered by the ruling regime.
Karbala, 80 kilometres (50 miles) south of Baghdad, is revered as being the site of the death of the great Shia martyr Imam Hussein, a grandson of the Prophet Muhammad.
Imam Hussein died 1,323 years ago and was thus denied the leadership of Muslims that Shia believe was his right.
In 680, he claimed the right to become the caliph or ruler, upon the death of Caliph Muawiya.
However Muawiya's son, Yazid, contested the claim and fighting resulted.
Saddam Hussein in Najaf in the late 1990s
Hussein's very small army was surrounded at Karbala on the edge of the desert and he was killed in battle.
His tomb, with a gilded dome and three minarets, is the central building in the city and a place of pilgrimage for Shia Muslims.
There is also a shrine to his brother Abbas, who also died in the battle.
Hussein's father, Imam Ali - the founder of what became the Shia movement - is buried at Najaf, 190 km south of the capital and 80 km from Karbala.
Ali married Muhammad's daughter, Fatima, and had expected to become the Muslim leader himself.
After Ali's death, the Shia then broke off from the Sunni form of Islam and have remained distinct to this day.
Najaf was once the power centre of Shia Islam, referred to as a world within a city. Under Ottoman rule, the city and its religious colleges were able to operate independently.
Karbala and Najaf are poor towns, which have traditionally made their money from the thousands of pilgrims - mainly from Iran and India - who have crowded into the tiny alleyways.
In 1991, after the Gulf War, Shia in Karbala, Najaf and elsewhere rose up in rebellion against Saddam Hussein.
However, the uprising was not supported by the United States and its allies, and the Iraqi Republican Guard was able to suppress it.
Tens of thousands of Shia across the south were killed, tortured and imprisoned and many fled to Iran.
The holy shrines in both cities were smashed by the tanks and artillery of government forces. They were, however, quickly restored using government funds.
The defeat of the uprising deeply alienated the Shia who make up some 60% of the country's population.
Since then, until its fall, the Iraqi Government reportedly restricted the twice-yearly pilgrimages to Karbala with ensuing clashes.
Shia parties have long been split by those who welcome American action and those who reject it.
Many Shia find distasteful the thought of American marines among the minarets in the holy cities.