2006: Sectarian War and the Awakening

Samarra
On February 22, 2006, bombers destroyed the golden dome of the al-Askari Mosque in Samarra, one of the most important Shia shrines. The attack unleashed waves of sectarian killing, especially in Baghdad, and the war's character changed.
Zarqawi
On June 7, 2006, an American airstrike killed Abu Musab al-Zarqawi, the leader of al-Qaeda in Iraq.
Ramadi and the Awakening
From March to November, the Army's 1st Brigade Combat Team, 1st Armored Division, known as the Ready First, fought for Ramadi, the capital of Anbar Province. Instead of operating from large bases, the brigade established small combat outposts throughout the city. At the same time, tribal leaders led by Sheikh Abdul Sattar Abu Risha turned against al-Qaeda in Iraq, launching the movement that became known as the Anbar Awakening. See Ramadi and the Awakening.
The end of the year
In December the bipartisan Iraq Study Group issued its report on the war. On December 30, 2006, Saddam Hussein was executed after his conviction by an Iraqi tribunal.