Central to the legacy of the US intervention in Iraq is the issue of sectarianism and the role it played in the reconstruction of the Iraqi state, as well as its impact on Iraqi society. Sectarianism was not a problem that emerged in post-2003 Iraq; its origins pre-date the modern Iraqi state and its relevance grew as a consequence of authoritarian rule and the emergence of parties and movements that drew their legitimacy and support from different ethno-sectarian communities. Events after 2003 exacerbated sectarian tensions and their significance for the political process and the Iraqi people. The 2006 sectarian civil war, the contestation of elections on the basis of identity and sect and continued sectarian violence are all symptoms of the post-2003 rise of sectarianism.

Sectarian polarisation in Iraq has also manifested itself on a regional scale. The removal of the Baath regime in 2003 has made a marked contribution to the polarisation of the Middle East along sectarian boundaries. Iraq's regional neighbours waged war on one another via proxies in Iraq. Jordan and Saudi Arabia, along with the Gulf states, opposed the replacement of Saddam Hussein's regime because of fears this would empower the Shias in Iraq and, therefore, Iran's influence in the region. To this day, Iraq's regional neighbours continue to influence rival political blocs and movements, prolonging the domestic polarisation in the country. Iraq's impact on the sectarian polarisation of the region was evident in the Arab Spring uprisings – particularly the civil war in Syria, which has transformed into a second front for Iraq's ongoing sectarian conflict. The overlapping and powerful combination of domestic and regional sectarian mobilisation means that sectarianism will continue to define the Middle East in the near and distant future.

The question of sectarianism and its contribution to the Iraqi state and society is a complicated one and, in addition to regional, domestic and pre- and post-2003 factors in Iraq, also encompasses other important questions including the role of symbolism, culture and geography. As such, the purpose of this chapter is to focus on the evolution of sectarianism in post-2003 Iraq, looking in particular at the issue of Sunni–Shia political relations and how sectarianism in the formative stages of the new Iraq has come to define the Iraqi state and society today.