When George Kennan wrote his famous "long telegram" he was making an important argument about why containment was necessary and why it could be successful.
He said the containment was necessary because the Soviets believed that they were being encircled by a hostile West. He said that they would try to expand as much as they could because they felt that they were in a perpetual conflict with the West.
Kennan argued that containment would work because the Soviet Union was weak. He said that it would be likely to experience instability within the country because it did not have enough of an economy to give its people all the things they wanted. Therefore, he argued, the West could contain the Soviets and wait for them to fall apart from within.