自上世紀90年代以來，美國的南海政策經曆數度調整。其大致脈絡是保持中立、有限介入、積極介入。而其背景，一方面，是伴隨着中國綜合國力不斷增強和海上力量的相對發展，以及中國南海政策日益清晰和由其主導的南海秩序開始形成，美亞太再平衡戰略在不斷推進的同時對中國的防範和牽制力度逐步加大。另一方面，是美國的南海保持中立政策名存實亡，在南海爭端中越陷越深，而中美兩國在南海的較量似乎也到了前所未有的劍拔弩張的程度。回顧近年來美國南海政策的調整與演變，可以用“三得三憂”來形容和概括。

衛星圖像顯示南沙群島的永暑礁是中國南海的一個土地復墾項目。中國建設的土地復墾工程在南沙群島的一些島嶼和珊瑚礁上將會按計劃完成。

對於“三得”，一是支持菲律賓向聯合國海洋法庭提交仲裁案，讓中國直接面對國際多邊機制的挑戰和壓力。2013年1月，在美國的公開支持下，菲律賓違背中國的意願和自己在雙邊和多邊協定中作出的通過友好協商和談判解決南海爭議的承諾，向聯合國海洋法庭單方面提起針對中國的強制仲裁程序。

二是力挺菲律賓堅守仁愛礁，為菲律賓試圖長期控制仁愛礁提供道義和實際支持。因為維持中菲南海島礁主權之爭不僅符合美國的利益，也有利於美國亞太再平衡戰略在南海周邊地區的推進和加快在南海針對中國的軍事部署。

三是借中建南“981”鑽井平台事件，一方面支持越南在西沙問題上和中國無理取鬧、挑起新的爭端，另一方面實現解除對越南的非殺傷性武器的禁售。越南與中國在南海上的糾紛，正好為美國利用南海問題遏制中國提供了契機。

美國在南海問題上除了收穫“三得”之外，還有其始料未及、頗感困惑的“三憂”：

一是中國將主導南海海上規則的制定。從上世紀90年代起，中國和東盟就啟動南海行為準則的磋商達成共識。2002年，中國和東盟簽署南海行為宣言（DOC）。2011年，中國與東盟達成落實南海行為宣言指針。2013年的中國東盟高官會議就加快南海行為準則的磋商達成新的共識。2014年，中國又提出了“南海爭端由當事方通過協商談判的方式解決，南海的和平穩定由中國和東盟共同維護”的雙軌思路。由此可見，未來南海海上規則制定的主導權似乎牢牢地控制在中國人手中了。

二是中國控制南海海權。伴隨着全球範圍內以海洋為中心的地緣政治博弈日趨激烈、美國亞太再平衡戰略的持續推進，以及沿海國開發海洋資源由近海向遠海推進，南海爭議已經由原來的爭端國之間的島礁主權和海洋管轄權爭議演變成了爭端國和利益攸關方圍繞地緣政治競爭、自然資源開發和航道管控等領域的激烈博弈。而中美兩國在南海的較量已經由初期的航行自由關切推進到國際法領域的爭端解決機制、南海航行和飛越制度，以及南海斷續線的法律地位等領域的交鋒和鬥爭。

三是中國宣布南海防空識別區。自從2013年11月中國宣布東海防空識別區以後，美國及整個國際社會要麼擔憂要麼預言中國將很快宣布設立南海防空識別區。為了回應國際社會的關切，中國官方多次明確表示，目前沒有宣布南海防空識別區的必要，雖然宣布南海防空識別區是中國主權範圍內的事，將來是否宣布則取決於中國對未來南海安全形勢的評估。目前中國在南沙地區進行的島礁建設再次觸動了美國的這根敏感神經。從某種意義上講，美國真正擔心的並非中國填島的速度和規模，而是揣摩中國填島背後的戰略意圖可能是為未來宣布南海防空識別區作必要的準備。

南海是中國國家安全的天然屏障、重要的海上戰略通道和實現海洋強國戰略的必爭、必保之地。而由於地緣政治和地緣戰略因素，對美國來說，控制南海、扼守南海和維持在南海的軍事存在，是維持其戰後形成的以雙邊聯盟為基礎的亞太主導權不可分割的組成部分。因此從這個意義上來說，中美兩國在南海的較量和爭奪是結構性的、戰略性的和不可調和的。

今天的南海，中美兩個大國突然成了主角。美國強勢介入，中國強勢維權，南海海域再次成為地緣政治的角力場。人們已經發現，由於美國亞太再平衡戰略的實施和南海政策的調整，南海問題已不再僅僅是中國和其他幾個爭端國之間的島礁主權和海域管轄權之爭，而是以中美南海矛盾為主、中國和其他聲索方的矛盾為輔的，既涉及地緣政治利益、海權和航行制度，又涉及島礁主權、海洋權益和資源開發的複雜爭議。與此同時，南海問題也將成為今後中美雙邊關係中的一個無法迴避的重要問題。因此，管控中美南海分歧、防止中美南海衝突而損害兩國關係，是中美高層決策者當前面臨的一個現實而緊迫的課題。

管控和防止中美兩國在南海的衝突不僅事關南海地區的和平穩定，而且關係到中美兩國關係的健康發展，以及整個亞太地區的長治久安。從美國方面來講，就是應該理解和照顧中國在南海的關切和利益，切實履行自己在南海問題上保持中立的承諾，對在南海針對中國的抵近偵察保持克制，以實際行動證明美國無意利用南海問題牽制中國。從中國方面來講，就是應尊重美國依據國際法在南海享有的航行和飛越自由，盡量避免單方面宣布可能衝擊中美政治互信和安全合作的南海防空識別區，切實加快南海行為準則磋商，以解決南海地區安全機制和危機管控機制缺失的問題，在南沙島礁建設上優先考慮民事功能，對超出自身防衛需要的軍事設施建設保持謹慎。此外，從機制建設角度講，中美兩國可以考慮在戰略經濟對話框架下建立一軌或一軌半的中美海洋問題磋商工作組，就管控中美海上危機、建立中美和諧海洋夥伴關係提出對策建議。條件成熟時，也可以考慮建立兩國海軍首腦、以及美太平洋艦隊司令和解放軍南海艦隊司令之間的熱線電話。

U.S. South China Sea policies have undergone several rounds of profound changes since the end of the 1990s, evolving from “staying neutral” to “limited intervention”, and then “active intervention.” This is because, on one hand, with Chinese comprehensive national strength continuously rising and its relative capabilities growing at sea, Chinese policies regarding the South China Sea have become increasingly clear, and a maritime order in the South China Sea dominated by China is beginning to take shape. While pressing ahead with its pivot to the Asia-Pacific, the U.S. is gradually enhancing its preventive containment of China. On the other hand, the U.S. is reducing its policy of “neutrality” into a nominal existence, getting involved more and more deeply in maritime disputes in the South China Sea, and China-U.S. rivalry in the area is unprecedentedly tense. In retrospect, adjustments and evolution of Washington's South China Sea policies in recent years can be summed up as three gains and three worries.

Satellite image shows the Yongshu reef of the Nansha Islands after a land reclamation project in south China. The land reclamation project of China’s construction on some stationed islands and reefs of the Nansha Islands will be completed as planned soon.

The first gain is, by supporting the Philippines quest to submit its maritime dispute with China to the International Tribunal for The Law of The Sea, it has subjected China to direct challenge and pressure from international multilateral mechanisms. In January 2013, with the open support of the U.S., the Philippines unilaterally proposed the compulsory arbitration procedure against China, against both China's will and the commitment Manila had made in bilateral and multilateral agreements to solving the South China Sea dispute through friendly consultations and negotiations.

The second gain is, the U.S. has adamantly supported the Philippines in “safeguarding” the Ren'ai Reef, providing moral and practical support for the latter's attempt to permanently control it. This is because maintaining the China-Philippines territorial dispute in the South China Sea is not only consistent with U.S. interests, but also facilitates its rebalancing to the periphery of the South China Sea and supports its military deployments against China.

The third gain is, taking advantage of the dispute over oil rig 981, it on one hand supported Vietnam to create new troubles for China on the Xisha Islands issue, and on the other hand actually lifted its lethal weapons embargo against Vietnam. The China-Vietnam dispute in the South China Sea has provided opportunities for the U.S. to contain China.

Besides harvesting the three gains on the South China Sea issue, the U.S. also has absorbed three unexpected worries.

First, it is worried that China will dominate rule-making in the South China Sea. Since the 1990s, China and the Association of Southeastern Asian Nations have reached consensus on launching consultations about a code of conduct in the South China Sea. In 2002, the two parties signed the Declaration on the Conduct of Parties in the South China Sea (DOC); in 2011, they signed the Guidelines for the Implementation of the DOC; in 2013, the two sides reached a new consensus on accelerating consultations about the proposed Code of Conduct in the South China Sea; in 2014, China put forward the dual-track approach that “disputes over the South China Sea be resolved through consultation and negotiation between disputing parties, peace and stability of the South China Sea be jointly preserved by China and the ASEAN.” It is thus evident that China may firmly dominate future rule-making regarding the South China Sea.

Second, it is worried that China may obtain overwhelming sea power in the South China Sea. With global geopolitical gaming centered around the oceans getting increasingly fierce, the U.S. pressing ahead with its strategic pivot to the Asia-Pacific, and exploitation of maritime resources by coastal nations extending from offshore to the high seas, disputes in the South China Sea have evolved from those between claimant countries over sovereignty and maritime jurisdiction over islands and reefs to fierce gaming in such areas as geopolitical competition, natural resources exploitation, and waterway control among claimant nations and stake-holding parties. China-U.S. competition in the South China Sea has thus upgraded from the previous concerns about freedom of navigation to those regarding dispute-resolution mechanisms, rules on navigation and over-flight in the South China Sea, and the legal status of the nine-dash-line in the South China Sea.

Third, the international community worries that China may declare an Air Defense Identification Zone in the South China Sea. Since China declared its East China Sea Air Defense Identification Zone in November 2013, the U.S. and the international community have either worried or predicted that it will soon announce an ADIZ for the South China Sea. China has repeatedly stated there is no need for such a zone for the time being. Though such action is within Chinese sovereignty, whether or not it will declare a new ADIZ will depend on its evaluation of security conditions in the South China Sea. China's ongoing construction activities on isles and reefs in the Nansha area have again struck American nerves. In a certain sense, it is more concerned about the possibility of the consequent declaration of the feared ADIZ than about the speed or scale of such construction projects.

The South China Sea is a natural line of defense for Chinese national security, an important strategic waterway, and a strategic must-have for it to become a maritime power. For the U.S., controlling the South China Sea and maintaining its presence there is indispensable for its dominance in the Asia-Pacific based on its bilateral alliances formulated in the post-war era. In this sense, China-U.S. competition and rivalry in the South China Sea is structural, strategic, and irreconcilable.

Given the implementation of the U.S. pivot to the Asia-Pacific and adjustments in its South China Sea policies, the South China Sea issue is no longer solely disputes between China and several other claimant nations about sovereignty over islands and reefs and maritime jurisdiction. It has become a very complicated one with China-U.S. conflicts assuming center stage, and those between China and other claimants in the background. Meanwhile, the South China Sea will be an important and unavoidable issue in China-U.S. relations. Therefore, managing China-U.S. disagreements over the South China Sea and preventing conflicts there from damaging bilateral ties is a practical imperative for decision-makers in both countries.

The U.S. should understand and take into consideration Chinese concerns and interests in the South China Sea, honor its pledge of “neutrality” on the issue, restrain its close-in surveillance and reconnaissance flights against China, and prove with actions its claim that it has no intention to contain China with the South China Sea issue. China should respect U.S. freedom of navigation and over-flight in the South China Sea based on international law, try its best to avoid unilaterally declaring the anticipated South China Sea ADIZ that may undermine political mutual trust and security cooperation, truly speed up consultations on the code of conduct so as to deal with the absence of security and crisis control mechanisms in the South China Sea, prioritize civilian functions in construction activities on the isles and reefs there, and be prudent about military facilities that go beyond defense purposes. From the perspective of mechanism building, the two parties can consider setting up a single or one-and-half track “China-U.S. working group for consultations on maritime issues” under their Strategic and Economic Dialogue. When conditions are ripe, they can also consider hotlines between naval commanders.