China allowed Soviet military aid passage through the country, but once the threat of having an American client cf. South Korea was waning (early 70s) repaired relations with the US and invaded Vietnam (1978).
Of course. Actually they have had many invasions and multiple occupations from China over the course of the last 10-20 centuries. They are, long-term historically speaking, essentially a China breakaway, but similarly southward-expansionist state.
Right now there is conflict with China over their Vietnam's territorial waters and the South China Sea. China invaded Vietnam via land as recently as 1979.
> They are, long-term historically speaking, essentially a China breakaway
Hmmm ... China hasn't controlled Vietnam in a very long time, AFAIK; maybe never. Vietnam has a different culture and language.
> similarly southward-expansionist state
South of Vietnam is ocean; what does this comment mean?
China has occupied Vietnam many times. See http://en.wikipedia.org/wiki/History_of_Vietnam
Vietnam has a different culture and language.
The language is very much a dialect of ancient southern Chinese. In fact, it is used by linguists to reconstruct (along with other languages) ancient Chinese pronunciation. Culturally, Vietnam has Chinese holidays like Chinese New Year, plus Chinese characters (though they are forgetting how to read them after the installation of Romanized script), Confucianism, Taoism, etc.
South of Vietnam is ocean; what does this comment mean?
Sinified Vietnam (ie. the first beginnings of modern Vietnam proper) expanded southward overrunning completely culturally distinct countries such as Champa, see https://en.wikipedia.org/wiki/Champa
You say that as if Vietnam did it on their own. They were as supported by China and Russia as Taiwan was by the US.
And now that China is encroaching on Vietnam's sovereignty, who do they cozy up to? The US.
It's all politics.