自上世纪90年代以来，美国的南海政策经历数度调整。其大致脉络是保持中立、有限介入、积极介入。而其背景，一方面，是伴随着中国综合国力不断增强和海上力量的相对发展，以及中国南海政策日益清晰和由其主导的南海秩序开始形成，美亚太再平衡战略在不断推进的同时对中国的防范和牵制力度逐步加大。另一方面，是美国的南海保持中立政策名存实亡，在南海争端中越陷越深，而中美两国在南海的较量似乎也到了前所未有的剑拔弩张的程度。回顾近年来美国南海政策的调整与演变，可以用“三得三忧”来形容和概括。

卫星图像显示南沙群岛的永暑礁是中国南海的一个土地复垦项目。中国建设的土地复垦工程在南沙群岛的一些岛屿和珊瑚礁上将会按计划完成。

对于“三得”，一是支持菲律宾向联合国海洋法庭提交仲裁案，让中国直接面对国际多边机制的挑战和压力。2013年1月，在美国的公开支持下，菲律宾违背中国的意愿和自己在双边和多边协定中作出的通过友好协商和谈判解决南海争议的承诺，向联合国海洋法庭单方面提起针对中国的强制仲裁程序。

二是力挺菲律宾坚守仁爱礁，为菲律宾试图长期控制仁爱礁提供道义和实际支持。因为维持中菲南海岛礁主权之争不仅符合美国的利益，也有利于美国亚太再平衡战略在南海周边地区的推进和加快在南海针对中国的军事部署。

三是借中建南“981”钻井平台事件，一方面支持越南在西沙问题上和中国无理取闹、挑起新的争端，另一方面实现解除对越南的非杀伤性武器的禁售。越南与中国在南海上的纠纷，正好为美国利用南海问题遏制中国提供了契机。

美国在南海问题上除了收获“三得”之外，还有其始料未及、颇感困惑的“三忧”：

一是中国将主导南海海上规则的制定。从上世纪90年代起，中国和东盟就启动南海行为准则的磋商达成共识。2002年，中国和东盟签署南海行为宣言（DOC）。2011年，中国与东盟达成落实南海行为宣言指针。2013年的中国东盟高官会议就加快南海行为准则的磋商达成新的共识。2014年，中国又提出了“南海争端由当事方通过协商谈判的方式解决，南海的和平稳定由中国和东盟共同维护”的双轨思路。由此可见，未来南海海上规则制定的主导权似乎牢牢地控制在中国人手中了。

二是中国控制南海海权。伴随着全球范围内以海洋为中心的地缘政治博弈日趋激烈、美国亚太再平衡战略的持续推进，以及沿海国开发海洋资源由近海向远海推进，南海争议已经由原来的争端国之间的岛礁主权和海洋管辖权争议演变成了争端国和利益攸关方围绕地缘政治竞争、自然资源开发和航道管控等领域的激烈博弈。而中美两国在南海的较量已经由初期的航行自由关切推进到国际法领域的争端解决机制、南海航行和飞越制度，以及南海断续线的法律地位等领域的交锋和斗争。

三是中国宣布南海防空识别区。自从2013年11月中国宣布东海防空识别区以后，美国及整个国际社会要么担忧要么预言中国将很快宣布设立南海防空识别区。为了回应国际社会的关切，中国官方多次明确表示，目前没有宣布南海防空识别区的必要，虽然宣布南海防空识别区是中国主权范围内的事，将来是否宣布则取决于中国对未来南海安全形势的评估。目前中国在南沙地区进行的岛礁建设再次触动了美国的这根敏感神经。从某种意义上讲，美国真正担心的并非中国填岛的速度和规模，而是揣摩中国填岛背后的战略意图可能是为未来宣布南海防空识别区作必要的准备。

南海是中国国家安全的天然屏障、重要的海上战略通道和实现海洋强国战略的必争、必保之地。而由于地缘政治和地缘战略因素，对美国来说，控制南海、扼守南海和维持在南海的军事存在，是维持其战后形成的以双边联盟为基础的亚太主导权不可分割的组成部分。因此从这个意义上来说，中美两国在南海的较量和争夺是结构性的、战略性的和不可调和的。

今天的南海，中美两个大国突然成了主角。美国强势介入，中国强势维权，南海海域再次成为地缘政治的角力场。人们已经发现，由于美国亚太再平衡战略的实施和南海政策的调整，南海问题已不再仅仅是中国和其他几个争端国之间的岛礁主权和海域管辖权之争，而是以中美南海矛盾为主、中国和其他声索方的矛盾为辅的，既涉及地缘政治利益、海权和航行制度，又涉及岛礁主权、海洋权益和资源开发的复杂争议。与此同时，南海问题也将成为今后中美双边关系中的一个无法回避的重要问题。因此，管控中美南海分歧、防止中美南海冲突而损害两国关系，是中美高层决策者当前面临的一个现实而紧迫的课题。

管控和防止中美两国在南海的冲突不仅事关南海地区的和平稳定，而且关系到中美两国关系的健康发展，以及整个亚太地区的长治久安。从美国方面来讲，就是应该理解和照顾中国在南海的关切和利益，切实履行自己在南海问题上保持中立的承诺，对在南海针对中国的抵近侦察保持克制，以实际行动证明美国无意利用南海问题牵制中国。从中国方面来讲，就是应尊重美国依据国际法在南海享有的航行和飞越自由，尽量避免单方面宣布可能冲击中美政治互信和安全合作的南海防空识别区，切实加快南海行为准则磋商，以解决南海地区安全机制和危机管控机制缺失的问题，在南沙岛礁建设上优先考虑民事功能，对超出自身防卫需要的军事设施建设保持谨慎。此外，从机制建设角度讲，中美两国可以考虑在战略经济对话框架下建立一轨或一轨半的中美海洋问题磋商工作组，就管控中美海上危机、建立中美和谐海洋伙伴关系提出对策建议。条件成熟时，也可以考虑建立两国海军首脑、以及美太平洋舰队司令和解放军南海舰队司令之间的热线电话。

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.