At one level, US President Donald Trump’s September 23 speech to the United Nations General Assembly was a rambling, at times barely coherent mix of rants and ramble, full of self-congratulation, abuse of allies, and plenty of plain, simple lies about much of the rest of the world. On another, it contained a warning to Asia and the world that America has left and gone for good.Here was the grandson of German and Scots immigrants to the US exhibiting to the world and to non-white America his strong distaste for “invaders” from what he earlier called “shithole” countries – non-western or non-Christian ones. Here was an oracle making up stories on climate issues, on Sharia law coming to London, and why he should get the Nobel Peace prize for ending multiple wars.Some might well dismiss this as the unhinged rhetoric of a megalomaniac. It would pass, normalcy would return. Yet it may also be seen as a window into Trump’s soul and those of tens of millions of his star-struck followers, one reacting to the relative decline of America by making out that it was now richer and stronger than ever, thanks to him. Much of the rest of the world was going to hell, overrun by immigrants and impoverished by believing fictions such as climate change.Saving America from such ills required firm government, intolerance of enemies as had flourished under Presidents Joe Biden and Barack Obama, and keeping America distant from the destructive influences from outside, be it a Muslim mayor in London or the “colossal invasion” across America’s southern border which he had stopped. US interests required such hitherto shocking developments ranging from a claim on Greenland, to the destruction of Venezuelan vessels allegedly and without evidence carrying drugs, and repeated support for Israeli President Binyamin Netanyahu’s genocidal Gaza destruction.All these things could perhaps be dismissed as temporary aberrations of one individual. Yet the reality is that they all enjoyed the (sometimes grudging) support of the Republican majority in Congress and of a Supreme Court majority supplied by the arch-conservative Federalist Society, swept up by the executive’s demands for ever larger powers, overriding congressionally approved policies and budgets.Free speech, at least when it criticized Trump and his policies, was undermined, critics were threatened with prison, academic freedoms were limited by the withdrawal of funds while Trump’s followers stepped up their barely disguised vitriol against those who did not adhere to the white, Christian self-image of Trump and his followers.Trump may not carry the majority with him. The US is too diverse for that. But the power of his MAGA followers remains in the ascendant, at least for now, meanwhile bolstered by, among other factors, a stock market which has (at least as far as AI and tech stocks are concerned) reached delusional valuations. It is no coincidence that the tech titans now align with Trump.But even assuming that that axis falls apart, that the great Trump boom turns to bust, that the tariff chaos gives way to stabilization at more moderate rates, that the US economy stagnates while inflation remains, that midterm elections 13 months away see a Democrat revival, there can be no undoing of all of Trump’s destruction.That is particularly the case with external relations and US foreign influences. Some policies may be reversed, renewed commitments made by a new administration to open trade, and plans to force local production of certain goods abandoned. Easing of immigration restrictions and anti-migrant rhetoric may provide a new boost to an economy which historically thrived on migration.Some ground lost can never be regained. The idea that the US is a reliable trade partner which operates under broadly understood rules, whether those are within the World Trade Organization or subject to specific mutually-agreed bilateral agreements, is being shaken to its core.Likewise, the dominance of the US dollar in international trade and finance has been seriously undermined. The dollar was probably destined to gradually lose ground to other currencies. Its weaponization against Russia following the Ukraine invasion caused concerns among several countries with large dollar reserve assets. But much more damage has since been done by Trump’s abrogation of trade deals and resort to strong-arm tactics of various sorts as the US makes every country, often friends more than foes, subject to transactional bilateral deals based on Trump’s whims and prejudices.At the same time, faith in the dollar has been eroded by Trump’s commitment to tax cuts which are almost certain to increase government debt and keep long-term interest rates high or see a sharp devaluation of the dollar. Trump’s campaign against the Federal Reserve and in favor of a looser monetary policy may suit his short-term domestic objectives but will encourage holders of US debt to seek alternatives.China is likely to gradually make yuan assets more readily available. The encouragement of cryptocurrencies by Trump may also see the rise of cryptos linked to currencies other than the dollar. Some in the US may view a reduced role for the dollar as neutral or even a benefit. However, it also reduces the inordinate privilege the currency has enjoyed and enabled the US to run decades of external deficits without feeling much cost.Most significant for the longer term of all aspects of Trumpism is a lack of trust in military alliances. Americans can be forgiven for suspicion of a global role when overconfidence leads them into Vietnam, Iraq, and Afghanistan. But Trump’s accommodation of Russian despot Vladimir Putin and failure to give adequate arms support to Ukraine is seeing Europe think again about the trans-Atlantic relationship. His ambiguity towards Taiwan worries not just Taiwan but Japan, Korea, Australia, and the Philippines. His attitude towards Modi and India defies explanation. Meanwhile, China continues to erode its firepower imbalance with the US.In short, empires rely on a mix of power, trust, and self-interest. Possibly, it is in US interest to retreat to the Americas. But this is not the 19th century. The Monroe Doctrine long ceased to have any meaning. America’s strength now is commercial and technological but alliances, whether mercantile or military, are fundamental to the global success of US enterprise. Friends regret America’s exercise in self-harm, a Brexit times 10. Like Brexit, it will be regretted. But returning to the previous status quo is not possible.