From “A Peace to End all Peace”, by David Fromkin:

Britain had never gone ahead with the notion of an American Mandate for Palestine but had proposed that the United States should assume the League of Nations Mandates to occupy and govern portions of Anatolia, Constantinople, the Dardanelles, Armenia, and the Caucasus. In the end those were narrowed down to Constantinople, the Dardanelles, and Armenia.

There were two reasons why Britain wanted the United States to assume these Mandates: it would implicate the United States in the Middle Eastern settlement so as the insure that she would help to support its terms; and it would station the United States on the front lines if Soviet Russia were ever to attack Turkey.