I am surprised that some people cannot see the big picture, or even listen to Truman's speech announcing that he had fired MacArthur and explaining the big picture.
The Berlin blockade had taken place in 1949 as had Tito's split with Moscow. Stalin was rearming the Warsaw Pact countries like mad with an invasion of Yugoslavia next on Stalin's list. (The West knew this).
The USSR did not have the wherewithall to hit America, but they certainly had the muscle to roll into West Germany or bomb US assets in Europe, especially Britain- which is why the US loaned the UK 70 B-29 bombers in early 1950.
The USSR had only supplied a handful of Soviet pilots secretly to serve in North Korea, an attack on China proper would almost certainly have brought a higher level and open Soviet assistance (or at least a lot more "volunteers"), thus expanding the war. US bases and the Navy vessels would have been fair-game targets.