Behind the Twitter storms and hyperbolic self-promotion, Donald Trump is nothing if not consistent in his approach to the world. A single aim runs through his international policy – to liberate himself and his country from constraints and to leave it free to pursue whatever he determines to be its destiny.

Thus, this month, he has rounded on allies in the G7 and formed a “partnership for peace” with Kim Jong-un. His complaints about how America has been ripped off may not be unjustified, as when they target China’s commercial behaviour or the low European contribution to defence. But in the broad sweep of the president’s view of the world, that is becoming rather beside the point as he sets a pattern of reaching for his six gun regardless of others, proclaiming victory come what may and gingering up his electoral base. The rest of the world has only been able to stand by, trying to absorb each bump as if they were isolated incidents. Trump’s shock-jock style obscures the way in which the global order of the past seven decades is being shifted amid the constant agitation from the White House and Air Force One.

Major players, above all China, are moving into the arena. Surveys show democracy in decline across the globe. Accepted rules lack force and international institutions are losing more of their influence. Alliances, along with the values meant to underpin them, become matters of short-term expediency as Palmerston’s observation about nations pursuing their interests meets Make America Great Again. Thus the G7 fracas and the way major US allies in Asia were not informed in advance about Trump’s decision to placate Kim by halting US-South Korean military exercises; as for personal diplomacy, Emmanuel Macron and Shinzo Abe can only wonder about their Bastille Day and golf course bromances with the president as he enthuses about Kim. Trump sees no reason for America to continue in the role it took up after the Second World War and celebrated five decades later with the collapse of the Soviet Union.

Although that has been plain in the president’s thinking since he hit the campaign trail, other countries do not appear to have appreciated the extent to which he means what he says and will translate it into action. His moves may seem disjointed and subject to abrupt short-term shifts as his humour alters, along with his calculation of what will play well with his public, as if seeking votes on a reality show. There is no “Trumpian order” and a new president in 2022 may revert to the old mean. But for the moment we can only go on what we have and, 18 months after his inauguration, it is clear that the self-styled “very stable genius” has ditched the vision of his predecessors since 1945 and that the rest of the G6 has no substitute while Putin nibbles at the edge of the western alliance and Xi Jinping lays out China’s increasingly global ambitions. The vision that Trump has abandoned was shaped by the belief that, after victory over the Axis powers, the US should play the global role it had declined after the Great War. Although Franklin Roosevelt remarked to Stalin that the GIs would soon be coming home, the division of Europe after Germany’s defeat made continuing involvement inevitable. The stationing of the troops was important, as was the formation of Nato and the air power to supply Berlin during the Soviet blockade.

US president Harry Truman, with Joseph Stalin and Winston Churchill, launched the Marshall Plan to rebuild Europe and save it from Soviet domination. Photograph: Bettmann/Bettmann Archive

However, the core American concern in the immediate postwar period was that the devastating legacy of the fighting might lead to the economic and social collapse of western Europe, opening the door to Soviet domination. Hence the Marshall Plan to revive the continent, or at least its western half, once Stalin had given the thumbs-down to participation by Soviet bloc countries. That gave the US presence a legitimacy that would not have come from simple military power and that played a role in encouraging co-operation that led to the original Common Market.

Europe was far from being the only part of the globe where developments in 1947-8 forged the future. But it is striking how major events, such as the independence of India and Pakistan, or the creation of Israel and the ensuing war, took place outside the rivalry between the US and USSR, while the civil war in China was not an international struggle. The global order that took shape in 13 crucial months of 1947-8 was, therefore, geographically limited, focused on events in Europe and reactions to them in Washington and Moscow. US-Soviet competition spread in due course (and the division of Korea had been formalised in 1948).

However, when democratic capitalism triumphed four decades later, it lacked the varied prescription for the wider world – and then tripped over itself in the new century as the spoils of prosperity were unevenly distributed and a complacent political class failed to react to the resulting resentments.

The US is not alone in replacing internationalism with an inward-looking mentality and transactional approach to international relations. But its role since it entered the Second World War in 1941 makes it crucial. Pax Americana rested for seven decades on internationalist alliances and institutions, generally observed rules and a political and economic system that, while serving US interests, offered advantages for its partners. Now the security and economic umbrella this provided – even if it sometimes came with downsides – is being rolled up, with implications that go well beyond individual decisions from the White House.

“The fundamental question of our time is whether the west has the will to survive,” Trump said last summer. “Do we have the confidence in our values to defend them at any cost?” Harry Truman asked a similar question when faced with Soviet power in the 1940s. His response was the Marshall Plan and all that flowed from it: the revival of Japan and Germany, internationalism and partnerships (even if it was clear who was in the driving seat) and encouragement of democracy (although support for dictators could be justified in the name of fighting communism). Today, the personality-driven response is very different in a fragmented world where combat is a sign of strength and it’s everyone for themselves.

Jonathan Fenby’s book, Crucible: Thirteen Months That Forged Our World, has just been published.