Vietnam signed mutual defense treaty with the Soviet, and Soviet was China's biggest thread at that time, this gesture was obviously hostile.
That's not what they call "turned against China".
Vietnam has been allying with Soviet long time ago.
It's not Vietnam who suddenly coop with Soviet in 1978.
It's China who ask Vietnam to deny Soviet since 1968-1969.
Once you argued like that, how do you think about the coop between China and USA, the ongoing enemy of Vietnam during 1971-1972. Could we call that turned against Vietnam ?
Soviet never harm China, even support tech to China. It's China who attempt to invade Soviet in 1969.
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1. Soviet support Vietnam until its separate and also support to China until 1969.
2. China support Vietnam and request Vietnam deny Soviet support since 1968-1969. ( to fight USA )
3. USA is fighting with Vietnam during 1971-1972, it's the hardest time of Vietnam War.
4. China fight with Soviet in 1969 and start cooperate with USA since 1971-1972 ( the ongoing enemy of Vietnam to which China support Vietnam fighting ).
Could we call that a turncoat ?
5. Vietnam signed the treaty with Soviet at the end of 1978.
China called that the turned against China ... ?