Truman by David McCullough:
Though it seemed so at the time, and would often be so presented in later accounts, the Truman Doctrine was not an abrupt, dramatic turn in American policy, but a declaration of principle. It was a continuation of a policy that had been evolving since Potsdam, its essence to be found in Kennan’s “Long Telegram” and in the more emphatic Clifford-Elsey Report. It could even be said that it began with Averell Harriman’s first meeting with Truman before Potsdam.
But, be that as it may, the Truman Doctrine would guide the foreign policy of the United States for another generation and more, for better or worse, despite any of the assurances by Acheson and Vandenberg that this was not the intent.