[tokenadult]

Several of the comments here relate to the Cold War uncertainty in the West about Warsaw Pact strategic intentions. Stalin's prompting North Korea's invasion of South Korea during the period of occupation following World War II (not to mention what happened to some of the countries of eastern Europe a while beforehand) is an adequate explanation for why the United States wasn't completely sure of Soviet intentions. The war in Korea made no sense, but Stalin started it for his own crazy reasons.