"In practice, as some in the region recall, long before the United States turned against them as part of its “pivot to Asia” in 2010, America had supported China’s claims in the Paracels and Spratlys. The U.S. Navy facilitated China’s replacement of Japan’s military presence in both island groups in 1945 because it considered that they were either part of Taiwan, as Japan had declared, or – in the words of the Cairo Declaration – among other “territories Japan [had] stolen from the Chinese” to “be restored to the Republic of China.” From 1969 to 1971, the United States operated a radar station in the Spratlys at Taiping Island, under the flag of the Republic of China.

“Neither the Paracels nor the Spratlys ever mattered to the United States at all (except as hazards to navigation) until they became symbols of Washington’s determination to curtail the rise of China’s power along its periphery. No country with claims to the Spratlys interferes with shipping or peacetime naval transit in the South China Sea. Nor does any party in the region have an interest in threatening commerce transiting it. The South China Sea is every littoral nation’s jugular. China and the other countries on the South China Sea have a far greater stake in assuring freedom of navigation in and through it than the United States does.

“It is not in the U.S. interest to perpetuate the antagonisms that now inflame relations between claimants in sections of the South China Sea. They poison Sino-American relations as well as other littoral states’ relations with China. China’s neighbors have to live with China, and China has to live with them."

http://chasfreeman.net/diplomacy-on-the-rocks-china-and-other-claimants-in-the-south-china-sea/

State Dept Chief of Spatial and Boundary Analysis, Daniel Dzurek – in his book «The Spratly Islands Dispute: Who's on First?», is also recounting how Japan returned the South China Sea to China in treaties that adhered to Japan's surrender agreement:

“Because the Allies, in particular the United Kingdom and the United States, could not agree on which government represented China, no Chinese delegation participated in the 1951 San Francisco Peace Conference. Therefore the Republic of China (Taiwan) negotiated a separate peace treaty with Japan, signed on 28 April 1952. Article 2 of the text included a reference to the San Francisco treaty:

““It is recognized that under Article 2 of the Treaty of Peace with Japan signed in the city of San Francisco in the United States of American on September 8 1951, Japan has renounced all right, title and claim to Taiwan (Formosa) and Penghu (the Pescadores) as well as the Spratly Islands and Paracel Islands””

Republic of China has argued that the explicit reference to the Spratly and Paracel islands in the text of this bilateral treaty implies Japanese recognition of Chinese sovereignty. Samuels and Lu have observed that, unlike the 1951 treaty, the Sino-Japanese text mentions the Spratly and Paracel islands in the same sentence as Taiwan and the Pescadores islands. The latter are generally recognized as Chinese territories. Moreover, according to the negotiating record Japan insisted that the renunciation article deal only with Chinese territory. This shows that the ROC and Japan viewed the islands of Taiwan, the Pescadores, the Spratlys, and tha Paracels as having a similar status – that is, belonging to China”

http://atimes.com/2016/09/china-says-japan-trying-to-confuse-south-china-sea-situation/