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!”

This article was first published in South China Morning Post.