How significant was Kennan’s ’long telegram’ for the creation of American Cold War strategy?

John Lewis Gaddis, The Cold War. Penguin Books 2005, 29:
"-- To say that it made an impact in Washington would be to put it mildly: Kennan's 'long telegram' became basis for the United States strategy toward the Soviet Union throughout the rest of the Cold War. Moscow's intransigence, Kennan insisted, resulted from nothing the west had done: instead it reflected the internal necessities of the Stalinist regime, and nothing the West could do within the foreseeable future would alter that fact. Soviet leaders had to treat the outside world as hostile because this provided the only excuse 'for the dictatorship without which they did not know how to rule, for cruelties they did not dare not to inflict, for sacrifices they felt bound to demand.' To expect concessions to be reciprocated was to be naïve: there would be no change in the Soviet Union's strategy until it encountered a sufficiently long string of failures to convince some future Kremlin leader - Kennan held out little hope that Stalin would ever see this - that nation's behavior was not advancing its interests. War would not be necessary to produce this result. What would be needed, as Kennan put it in a published version1 of this argument the following year, was a 'long-term, patient but firm and vigilant containment of Russian expansive tendencies.'"

1 "X" [George F. Kennan], ”The Sources of Soviet Conduct", Foreign Affairs, 25 (July 1947).
Kennan and Containment