Sectarian violence in the current conflict began to grow from early 2005. But the destruction of an important Shia shrine in February 2006 saw attacks between Sunni and Shia militias increase dramatically.
The fear caused by sectarian attacks has seen many Iraqi families abandon their homes and to move to other areas within the country that are dominated by their own religious or ethnic group.
The International Organization for Migration, who monitor numbers of displaced families, say most people are moving to be with friends and relations.
The United Nations' refugee body says the number of Iraqis living beyond the country's borders as refugees is about two million.
Many of these are living in Syria, Jordan, Egypt, Lebanon and the Gulf states. The UN says more than 300,000 Iraqis have returned home from Iran since 2003.
The total number of individuals displaced within Iraq since February 2006 has more than doubled in the first five months of 2007, taking the total number of internally displaced persons there to more than 2m.