What are the different religious groups in Iraq?

In 634 A.D., the newly-created Muslim empire expanded into the region of Iraq, which at the time was part of the Persian Empire. Muslim armies, under the command of Khalid ibn Waleed, moved into the region and defeated the Persians. In about 762 A.D., the Abbasid dynasty took over rule of the vast Muslim world and moved the capital to the newly-founded city of Baghdad.

What's the Difference Between Shia and Sunni Muslims?

Both Sunni and Shia Muslims share the most fundamental Islamic beliefs and articles of faith. The differences between these two main sub-groups within Islam initially stemmed not from spiritual differences, but political ones. Over the centuries, however, these political differences have spawned a number of varying practices and positions which have come to carry a spiritual significance.

The division between Shia and Sunni dates back to the death of the Prophet Muhammad, and the question of who was to take over the leadership of the Muslim nation. Sunni Muslims agree with the position taken by many of the Prophet's companions, that the new leader should be elected from among those capable of the job. This is what was done, and the Prophet Muhammad's close friend and advisor, Abu Bakr, became the first Caliph of the Islamic nation. The word 'Sunni' in Arabic comes from a word meaning 'one who follows the traditions of the Prophet.'...

The Shia Muslims believe that following the Prophet Muhammad's death, leadership should have passed directly to his cousin/son-in-law, Ali. Throughout history, Shia Muslims have not recognized the authority of elected Muslim leaders, choosing instead to follow a line of Imams which they believe have been appointed by the Prophet Muhammad or God Himself. The word 'Shia' in Arabic means a group or supportive party of people. The commonly-known term is shortened from the historical 'Shia-t-Ali,' or 'the Party of Ali.' They are also known as followers of 'Ahl-al-Bayt' or 'People of the Household' (of the Prophet)...

It is important to remember that despite all of these differences in opinion and practice, Shia and Sunni Muslims share the main articles of Islamic belief and are considered by most to be brethren in faith. In fact, most Muslims do not distinguish themselves by claiming membership in any particular group, but prefer to call themselves simply, 'Muslims.'"