This Analysis in The Times of London points out that it was during a phone call that president Trump made to president Erdogan of Turkey that the decision to withdraw from Turkey was made. The call was meant to caution Erdogan in attacking Kurdish forces in Syria. When Erdogan confronted Trump that he had said he would leave when ISIS was defeated, now that ISIS was defeated 99% why was he not withdrawing. At that point Trump asked his advisor Bolton if this was true and when he was told ISIS had been defeated Trump simply made up his own mind to withdraw to the shock of Pompeo, Mattis and Bolton, key persons in the defense and state departments.
In an earlier meeting Trump had told them they had 6 months time and the president now felt he was going to make his own decision.
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