LONDON • According to a well-established tradition, former presidents of the United States don't criticise their White House successor in public, regardless of how intensely they may disagree with him. Yet this honourable taboo - one of the few to have survived in US politics - has just been broken by former presidents George W. Bush and Barack Obama who, almost in unison, publicly tore into the policies of President Donald Trump.

Their scathing rebukes pursued different angles, but agreed on one fundamental point: that President Trump's determination to cast America's foreign and security policy as one purely in pursuit of what Mr Trump regards as US national interests risks "the return of isolationist sentiments, forgetting that American security is directly threatened by the chaos and despair of distant places", as Mr Bush put it.

It was a theme that Senator John McCain, that venerable voice of America's lawmakers, also echoed in a recent speech, in which he rejected Mr Trump's "half-baked, spurious nationalism".

However, it comes as no surprise that Mr Trump continues to regard criticism of his pursuit of purely national interests in foreign policy as a badge of honour. "I can understand how certain countries and the leaders of certain countries may feel" about America's new nationalist stance, he told a press conference during last week's visit to Washington by the Prime Minister of Greece. "But we're just not going to allow the United States to be taken advantage of by other countries any more, and there's nothing we can do about that", he added, while his Greek visitor politely pretended to be counting the flowers in the White House Rose Garden.

But the reality is that Mr Trump's tilt towards an "America First" foreign and security policy is far from exceptional, for it is accompanied by growing nationalist and populist tendencies in the foreign policies of many other key global players.

One thing is clear: should this process continue, it could fundamentally change the way international cooperation is handled; it may increasingly resemble the way states dealt with one another during the 19th century, rather than the way diplomacy has been conducted during our lifetime.

FOREIGN POLICY BEYOND NATIONAL INTERESTS

Many leaders through the ages have believed that foreign policy is nothing but an extension of national policy through other means; as Lord Palmerston, who served as Britain's prime minister twice during the 19th century and who used to believe that the best way of dealing with foreign countries is by unleashing on them his gunboats, once famously remarked that Britons had "no eternal allies and we have no perpetual enemies; only our interests are eternal and perpetual".

In that respect, therefore, Mr Trump conforms to a pattern that is older than the presence of his family on America's soil.

But it is also true that during the 20th century, and particularly since the end of World War II, the foreign policies of key countries have been animated by more than just a narrow expression of national interest. The creation of multilateral institutions from the United Nations to the financial structures of the Bretton Woods frameworks - the International Monetary Fund and the World Bank - and the General Agreement on Tariffs and Trade, which became the World Trade Organisation in 1995, is a case in point. So was the belief that open and transparent markets with minimal government intervention - the so-called Washington Consensus - should lay the foundation for global economic growth.

Of course, it is possible to dismiss all of this as just an ill-disguised bid for American-led global domination, and to a certain extent it was precisely that. But to see the whole period of the so-called liberal order as just a purely cynical Western domination gambit is to reduce history to parody.

For it was through the principles of upholding a global vision and of offering support, often with little hope for immediate repayment, that Europe as well as large swathes of Asia recovered from the destruction of World War II.

GLOBALISM BEYOND AMERICA

Without America's readiness to open its markets to free trade and suffer the consequences of trade competition, China would not have risen as quickly as it has done over the past four decades. And to this day, America provides, as Professor Joseph Nye recently remarked, "public goods such as freer trade and freedom of the seas" that allow other nations to grow, and often emerge out of America's shadows.

For instance, few people seem to be aware of the fact that, while the US shoulders the biggest burden of providing security in the Middle East, it is Asia which consumes the biggest share of the Middle East's oil and gas exports; the freedom of navigation for oil tankers heading to China is enforced by US Navy ships in the Gulf.

Nor is the pursuit of a globalist agenda that extends beyond narrow national interests a purely American phenomenon. The European Union's entire existence is justified by the image of the continent as a "good global citizen", a collection of states that supposedly left their nationalism behind in pursuit of a pooled sovereignty.

And other nations did the same: just think of the now-defunct Soviet Union's huge amount of assistance to many nations around the world, in the belief that this would result in the triumph of communism, or of the massive assistance that Mao Zedong's China gave to the developing world during the 1960s, often in pursuit of promoting another brand of communism. Much of this was misguided and wasted. But much was given as part of a global vision.

BACK TO PLAIN NATIONAL INTERESTS

Contrast this to the situation today. The US is ruled by a president who views the world as a jungle inhabited by beasts just waiting to bite at America, and sees himself as the only protector able to keep them at bay, by building walls and tariff barriers. And far from being an exception, Mr Trump is part of a trend.

In all their discussions about leaving the EU, not one British politician spoke about the security needs of Europe, and how these may be damaged by Britain's departure from the EU; the whole debate was about how the British may benefit, usually at the expense of others. A number of EU countries are now openly flouting Europe-wide obligations, with the argument that this could change their "national character" or threaten their very existence as a nation.

There is Russia, which now worships national interest as the highest form of existence, or Turkey, which sees no further need for making any sacrifices to its allies in Europe.

And, of course, there is China, where "patriotism" is a pivot around which almost everything revolves, and where millions of "Little Pink" Internet users regularly bombard foreign governments with kukoupoxin - or "grandmotherly advice" - each time any foreign leader dares to even question any aspect of official Chinese policy, sometimes with the direct sanction of China's ruling party.

A variety of reasons account for this process of "re-nationalisation" of foreign policy. The gradual decline in America's global dominance and the emergence of other power centres prompted countries to recalibrate their foreign and security policies, and national interests are, quite naturally, their first point of reference in this process.

The rise of populist political movements, supported by those who have failed to benefit from globalisation in many Western countries, also forces governments to retreat into their national shells. And the newly emerging global powers such as China, always in two minds over whether to preserve or replace the current global system, are now in a position to do either, but often do neither, thereby creating further tremors.

Be that as it may, if the process is not reversed, the consequences could be dire.

Multilateral institutions are already frozen: just look at the EU which, for the first time in its existence, is about to shrink in both size and scope; and at Asean, which finds it difficult to regain its stride.

Just look at the United Nations, where the US is in the business of curtailing its commitments. Free trade deals are being abandoned, and some may unravel.

But ultimately, the re-emergence of the pursuit of the purely national interest as the key driver of foreign and security policies may herald a step back to the world of the 19th century, one in which each government strives to steal a march on the other, viewing each concession as weakness and hailing each victory as a supreme vindication, unable to offer compromises because of nationalist clamour back home and unwilling to appear weak.

And we all know how that period ended.