我们在21世纪面临的最大挑战是什么？并非是中国的崛起，而是美国的衰落。
这种衰落尽管是相比较而言，但又显而易见。美国在全球GDP中所占的份额已从二战结束时估算的50%下降到1985年的22.4%，以及今日（2018年）的15.2%。国际货币基金组织（IMF）预测，到2023年，这一比例将下滑至13.9%。
美国的衰落始于特朗普就任总统之前，可以追溯到冷战结束时美国必胜主义的巅峰时期，并可能在特朗普之后持续衰落。他的“美国优先”议程正在加速这一进程。
退出条约、诋毁盟友、发起贸易战…在特朗普政府治下，美国看起来比任何其他国家都更像一个“修正主义大国”。
特朗普的出现是意外还是必然？当被问及对1789年法国大革命的看法时，中国总理周恩来有句名言：现在说还为时过早。
历史会对特朗普做出评判，但可能不会发生在两年或六年以内（编者注：文章刊登于2019年1月）。对于这位难以捉摸的美国总统来说，问责和公信力似乎并不重要，他更喜欢的是玩火。
即使是那些对中国最苛责的人也不得不承认，中国在过去40年中实现了和平崛起。但是美国的衰落，如果不可避免，也会同样是和平的吗？
首先，不要相信神话故事，让我们运用常识分析。人们不需要读保罗·肯尼迪的经典著作《大国的兴衰》，就能知道大国无时无刻不在兴起和衰落的过程中。撇开这些溢美之词，美国既不是“例外”也不是“不可或缺”，美国人也不是什么“几乎被上帝选中的民族”。
一个美国人不一定比一个马尔代夫人更为他的国家感到自豪，后者每天都受到来自世界各地游客的致意。如果人人生而平等，那么国家也是如此。
美国仍有充分理由保持自信。它拥有丰富的自然资源、迷人的文化、强大的教育体系、富有创造力的企业家们和有利的人口结构。
美国的军事力量在任何地方都不会受到挑战，并将长期保持下去。在许多全球问题上，世界仍指望美国参与，尽管不一定要其扮演领导角色。
如果生活的真谛是要做出正确的选择，那么美国就不需要有那么多“至关重要的国家利益”。它必须做选择，但不应以牺牲他国利益为代价推行单边主义和保护主义。
就此而言，美国从叙利亚或阿富汗撤军或许可以理解，但退出巴黎气候协定和伊朗核问题协议却令人费解，这两项协议攸关各方利益。
如果像特朗普去年（2018年）12月向驻伊美军宣布的那样，“美国不能继续做世界警察”，他就应该鼓励联合国发挥更大的作用。
要最好理解联合国的用处，可以问这些问题：如果没有联合国处理从冲突和贫困到社会和经济发展以及人权等一系列问题，世界会变成什么样？
尽管联合国有时因官僚主义和效率低下而受到批评，但它最能代表在全球问题上的集体决策和行动。
联合国设在美国。作为联合国的创始国之一和最大的会费分摊国，美国应该与其他国家一道改革联合国，而不是削弱这个世界上最大的政府间组织。
美国和中国这两个世界上最大的经济体之间的复杂关系是可以理解的，特别是考虑到人们普遍预测中国将在2030年左右在经济上超越美国。
对中国来说，面临的挑战是，在美国将其视为头号战略竞争对手的情况下，如何继续和平崛起。不管你认同与否，中华人民共和国一直保持着社会主义国家的身份。
但今天，它在当前的国际体系中变得更加强大和自在。北京方面愿与感兴趣的国家分享其经济成功的故事，不应被视为试图在海外宣传其意识形态。
即使中国按计划于2049年实现“中华民族伟大复兴”，中国人均收入仍将远低于美国。把21世纪描述为“中华统治下的和平”未免夸张。
美国将中国视为战略竞争对手并非正确的战略取向；相反，伴随自信的减弱和焦虑压倒理性，美国更像是迷失了方向。这两个巨人会不会糊里糊涂陷入一场看似不可能的冲突，比如在南中国海？
目前的情况类似于一种“非典型安全困境”，即美国—声称在南海问题上不持立场的第三方—已经介入，并与中国剑拔弩张，东盟则在旁焦急观望。
公平地说，北京和华盛顿都不希望发生冲突，但（2018年）9月份中美舰艇险些相撞，是对这种冲突风险的最新警示。
一个更强大的中国只会更坚决地击退在家门口的挑衅。因此，今天在海上或空中发生的任何碰撞，其后果都将远远超过2001年中美军机发生的致命碰撞事件。
而特朗普会怎样在推特上谈及这个？“糟糕透了！”
China has risen peacefully, but will America go down without a fight?
What is the greatest challenge we face in the 21st century? It is not China’s rise but America’s decline.
The decline, albeit relative, is obvious. The United States’ share of global GDP has dropped from an estimated 50 per cent at the end of the second war to 22.4 per cent in 1985 and to 15.2 per cent today. The International Monetary Fund expects it to slide to 13.9 per cent by 2023.
America’s decline started before Donald Trump became president, dating from the apogee of triumphalism at the end of the cold war, and will probably continue after him. But he is accelerating it with his “America first” agenda.
Pulling out of treaties, denigrating allies, starting trade wars: under the Trump administration, the US looks more like a “revisionist power” than any other nation.
But is Trump an accident or the inevitable? When asked what he thought of the French Revolution of 1789, Chinese premier Zhou Enlai famously said it was too soon to say.
History will judge Trump, but probably not in two or six years. Accountability and credibility don’t seem to matter to the unpredictable US president, who prefers to play with fire.
Even those most critical of China have to admit its rise in the past 40 years has been peaceful, but could America’s decline, if inevitable, be equally peaceful?
First, use common sense rather than believe in myths. One doesn’t need to read Paul Kennedy’s classic, The Rise and Fall of the Great Powers, to know great powers are rising and falling all the time.
Brushing aside the rhetoric, America is neither “exceptional” nor “indispensable”, nor are Americans God’s “almost chosen peoples”.
An American is not necessarily more proud of his country than a Maldivian who is greeted every day by tourists coming from all corners of the world. If all men are created equal, so are all countries.
The US still has good reason for self-confidence. It has rich natural resources, an attractive culture, a strong educational system, creative entrepreneurs and favourable demographics.
US military power is unchallenged anywhere and will remain so for a long time. The world still looks to the US for engagement, although not necessarily leadership, on many global issues.
If life is about making the right choices, then the US doesn’t need to have so many “vital national interests”. It has to be selective, but should not pursue unilateralism and protectionism at the cost of others.
In this regard, the US’ withdrawal of troops from Syria or Afghanistan may be understandable, but not its withdrawal from the Paris climate accord and the Iranian nuclear deal, pacts that have a bearing on all.
If “the United States cannot continue to be the policeman of the world”, as Trump declared in December to US troops in Iraq, he should encourage the United Nations to play a bigger role.
The usefulness of the UN can be best ascertained by asking: what would the world become without the UN taking care of issues ranging from conflict and poverty to social and economic development and human rights?
Although the UN is sometimes criticised for bureaucracy and low efficiency, it best represents collective decisions and actions on global issues.
The UN remains on American soil. As one of the creators of the UN and the largest financial contributor, the US should join others in reforming it, instead of weakening the largest intergovernmental organisation on the planet.
The relationship between the US and China, the world’s two largest economies, is understandably complicated, especially given that China is widely expected to overtake the US economically by around 2030.
For China, the challenge is how to continue rising peacefully when the US sees it as a top strategic competitor. Like it or not, the People’s Republic of China has retained its identity as a socialist country.
But today, it is much stronger and more comfortable with the current international system. Beijing’s offer to share its stories of economic success with those who are interested should not be taken as an attempt to promote its ideology abroad.
Even if China realises “the great rejuvenation of the Chinese nation” as planned in 2049, China’s per capita income will still be much lower than that of the US. It is an overstatement to describe the 21st century as Pax Sinica.
That America takes China to be a strategic competitor is not strategic orientation; rather, it looks like a loss of direction as self-confidence wanes and angst prevails over rationale. Could the two giants sleepwalk into a conflict, say, in the South China Sea?
The situation currently resembles an “atypical security dilemma” in that the US – a third party that claims to take no position on the South China Sea issue – has stepped in and is standing eyeball to eyeball with China, with Asean watching anxiously.
It is fair to say that neither Beijing nor Washington wants a conflict, but a near-collision between a Chinese and US naval vessel in September is the latest reminder of the risk.
A stronger China would only be more resolute in fighting off a provocation on its doorstep. Therefore, any collision at sea or in the air today would be far more consequential than the deadly collision between Chinese and US military aircraft in 2001.
How might Trump tweet about this? “Very bad!”
（翻译：中国论坛 廉晓诺 审校：中国论坛 韩桦、国佳）
