The Pentagon is disputing this letter. They say that they have no idea whatsoever where it came from or who authorized it:
U.S. Military Letter Announcing Potential Iraq Withdrawal ‘Was A Mistake,’ General Says
The Defense Department disputed the letter, which came to light a day after the Iraqi parliament voted to revoke the country’s invitation to host U.S. troops.
http://www.huffpost.com/entry/united-states-military-exit-iraq-parliament-vote_n_5e139219e4b0843d3616faac

OCU