Not only did it happen before nuclear weapons, but China has never been expansionist. The soldiers it sent into Korea were to help the north koreans. For a thousand years or more China has been content to stay within it's borders.
Both Generals LeMay ( USAF SAC ) and MacArthur advocated widespread nuclear response to the Chinese intervention. They were overruled by President Truman.
> For a thousand years or more China has been content to stay within it's borders
Are the Paracel Islands, Spratly Islands and Formosa within China's borders? To all of which China lays claim.
Why they've attacked Vietham then?