.
With or without America the Sunni and Shi'a will kill each other and those not quite Muslim enough.
Over the years, SunniShia relations have been marked by both cooperation and conflict. Sectarian violence persists to this day from Pakistan to Yemen and is a major element of friction throughout the Middle East.[5][6] Tensions between communities have intensified during power struggles, such as the Bahraini uprising, the Iraq War, and most recently the Syrian Civil War
Iraq-
Some of the worst sectarian strife ever has occurred after the start of the Iraq War, steadily building up to present
Egypt-
Almost all of Egypt's Muslims are Sunni,[102] but the Syrian Civil War has brought on an increase in anti-Shia rhetoric,[103] and what Human Rights Watch states is "anti-Shia hate speech by Salafis"
Pakistan-
Until recently ShiaSunni relations have been cordial, and majority of people of both sects participated in the creation the state of Pakistan in 1940s
Lebanon-
Though sectarian tensions in Lebanon were at their height during the Lebanese Civil War, the ShiaSunni relations were not the main conflict of the war.
United States-
In late 2006 or early 2007, in what journalist Seymour Hersh called The Redirection, the United States changed its policy in the Muslim world, shifting its support from the Shia to the Sunni, with the goal of "containing" Iran and as a by-product bolstering Sunni extremist groups
http://en.wikipedia.org/wiki/Shia–Sunni_relations
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