In January 1974, South Vietnamese forces clashed with the People’s Liberation Army Navy in the Paracel Island chain—a hot spot between China and present day Vietnam today.

The two countries had established their competing claims to the islands—which the Vietnamese refer to as Hoang Sa and the Chinese call Xisha—well before they came to blows. The South Vietnamese navy regularly chased foreign fisherman away in the 1960s under the umbrella of American military might.

But in 1973, the U.S. started cutting off support for the Vietnamese Republic. American negotiators cut deals with the North in Paris and Congress then cut aid to the South.

Beijing quickly quickly realized the situation was changing in their favor. Communist forces already controlled the northern half of the archipelago.

http://nationalinterest.org/blog/the-buzz/south-china-sea-fact-one-the-last-things-south-vietnam-did-17502