From wikipedia:

"History

In 2005, the Abu Mahals, a tribe that smuggled across the Syrian border, was being forced out of their territory by a tribe allied with Al Qaeda in Mesopotamia. The tribe proposed an alliance with the United States force in November 2005 and began receiving weapons and training.[9][1] In 2007 the leader of the movement, Sheik Abdul Sattar Buzaigh al-Rishawi, formed the Anbar Awakening Council also called "Anbar Awakening" to counter the influence of Al-Qaeda in Iraq.[9]

Sheik Abdul Sattar Buzaigh al-Rishawi was assassinated by a suicide bombing in September 2007.[10] His brother, Abdul-Jabbar Abu Risha, took over as leader, but so far has been unable to unite the various awakening militias.[1]"

Looks like the early date was adhoc and opportunistic, the latter date more formalized and broader based.