From 1944 to 1946, Kennan served as the deputy head of the US mission in Moscow, where he sent the famous "long telegram" (5,300 words) to the Secretary of State James Byrnes. The telegram outlined a new strategy for diplomacy with the Soviet Union, arguing that their foreign relations were heavily influenced by Stalin's manipulation of Russia's contemporary feeling of national insecurity to create the illusion of a hostile world which was essential to maintain his autocratic rule. For this reason, Kennan suggested that the US ought to support Western institutions so that they might resist Soviet pressure and therefore render it less powerful.
Kennan's "long telegram" caught the eye of Secretary of the Navy James Forrestral, who convinced him to publish his second strategic article publically under the pseudonym "X." "The Sources of Soviet Conduct" appeared in the July 1947 edition of Foreign Affairs and discussed Stalin's use of Marxism-Leninism to incite a revolution against the rest of the outside capitalist world, advising a policy of containment.
The article drew both criticism and attention, and Kennan's identity as author was soon revealed. As he saw his article taken as prescriptive for military strategy, Kennan joined the ranks of his own critics. He had tried to make a distinction between the Soviet Union an international Communist movements- a distinction he thought could be exploited to weaken Russian influence- and did not believe that Soviet expansion necessarily required US military intervention.
Kennan opposed the building of a hydrogen bomb, the rearmament of Germany, and the forcible unification of the Koreas, all of which aided his already waning influence over Secretary of State George Marshall's successor, Dean Acheson. After being replaced in 1950, he took up a visiting post at the Institute for Advanced Study at an invitation from Robert Oppenheimer. Kennan served as US Ambassador to the Soviet Union in 1952 and Ambassador to Yugoslavia from 1961 to 1963. He passed away at age 101 in March, 2005.