The three groups that turned to fighting and internal violence after the U.S. removal of Saddam Hussein's old Iraqi regime, were Sunnis, Kurds and Shias.
Under the leader Saddam Hussein, Iraq had a dictatorial regime that was out and out brutal in nature. The United States started Operation Red Dawn, where deposed Saddam Hussein and got done with his power over Iraq. He was later convicted of crimes against humanity and sentenced to death by hanging, by the Iraqi court.
The Al-Qaeda did not like this step, mostly because of the interference of United States in their state politics and thus they always tried to push it off their country.
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