How might those expansionist desires challenge the Western principle of national political self determination

Churchill believes the Soviet Union “desires the fruits of war and the indefinite expansion of their power and doctrines.” How might those expansionist desires challenge the Western principle of national political self determination, a cause it championed during World War 2?
Churchill’s speech acknowledges “Russia’s need to be secure on her western borders,” but at the same time it raises concerns about Soviet actions in Eastern Europe. Is Churchill being inconsistent? Or does he provide concrete justifications for those concerns?
In his speech, Churchill asserts “There is nothing they (the Russians) admire so much as strength, and nothing for which they have less respect for than military weakness.” If he isn’t advocating a direct military confrontation with the Soviet Union, then what is he saying?
Churchill delivered this speech to an American audience, but after reading it one might conclude it could have been given in any western country. Why did he pick the US?