What does the world do about Donald Trump, the most erratic, egocentric and compromised US president ever to hold office? Trade war or no trade war? Summit or no summit with North Korea? Is Nato obsolete or relevant? Military or no military involvement in the Middle East?

The US remains the overwhelmingly predominant economic, military and financial force on the planet. For 70 years, it has sustained a world order that, whatever its downsides, was at least an order with predictable patterns of international behaviour, along with a bias to openness, trade and peace. Suddenly it is all up in the air.

For the west’s enemies and critics, the new chaos is welcome. Russia’s Vladimir Putin and China’s Xi Jinping are united in their view that the postwar order was a victor’s order, organised to suit US interests and from which it has benefited hugely. China chafes under the supremacy of the dollar, the US’s network of military bases, its dominance of the great international institutions and the way global trade and financial flows centre on the US.

Russia still smarts from how the ring of satellite countries that constituted the Soviet Union have since detached themselves from Russian influence. Putin wants to level the score – witness his invasion of Crimea, his siege of Ukraine (involving, we now learn, the Russian shooting down of flight MH17, killing all 298 passengers) and his armies massing on the frontiers of the Baltic republics.

Both see yet more opportunities spinning out of Trump’s incredible stupidity in deliberately destabilising the very system from which the US benefits so much. His waywardness is offering the chance to create new Russian and Chinese spheres of influence. China is being given the chance to dominate Asia, Russia the chance to recreate the Soviet Union’s influence in its old borderlands. And both can do it while talking the language of peace and respect for obvious global menaces such as climate change. Has a bigger fool ever occupied the White House?

Trump’s fights and deals may reward the rich, but many of the disenfranchised rally to the philosophy of self-assertion

In Trumpland, however, both he and his “ base” see themselves as advancing America first. It is a universe that has created its own hermetically sealed reality. Trump’s long, rambling folksy speeches at his rallies – he uses “folks” a lot, diving into eccentric diversions at enormous length – are bound together by a combination of paranoia, vanity, prejudice and jokiness that is as curiously compelling as it is bonkers. But the unifying theme is that life is a zero-sum game: there are never gains from collaboration, no virtue in sharing and no value in anything but asserting your own will – with a gun, with a cheque book or with a missile. Give no quarter, look for the other side’s weakness and brutally cut the deal to walk away taller and prouder yourself. Trump’s fights and deals may reward the rich, but many of the disenfranchised rally to the philosophy of uncompromising self-assertion. It’s potent and it’s dangerous.

Thus the twists and turns in American foreign policy. Iran has to be seen to kowtow to US will. Palestinians must accept the new Middle Eastern reality: there will be a US embassy in Jerusalem. Thus China is threatened with swingeing tariffs to be suspended only after it blinks first. Nato is tolerated because members are increasing defence spending as demanded. The US will only re-enter the Paris climate change accords if the world reshapes it wholly to accommodate US interests. North Korea must overtly bend to US will if there is to be a summit. Foreign policy has become a series of zero-sum games: the US has to win each one.

This may work with Trump’s base – but it is a ludicrous way for the superpower to behave. For a start, it abandons any attempt to sustain world affairs systemically around a coherent set of shared principles or maintain the US’s leading role in its varying alliances. The world system, based on the dollar, Pax Americana and economic openness, is the foundation not only of global trade growth and prosperity since the Second World War but also of the US’s own economic model; it is why its hi-tech multinationals, rather than China’s, are world actors. By undermining the system, Trump will lose the benefits of systemic gain for tiny wins – and often not even for those.

Furthermore, it fragments the entire alliance network– and legitimises Russia and China. Thus Russia and the Europeans are acting in concert to sustain the Iranian denuclearisation deal. The EU is refusing a transatlantic trade deal until the US recommits to the Paris climate change agreement, with China and Russia positioning themselves as on the side of the angels. China may increase its purchases of US soya bean and oil – the reason why the trade war was suspended – but it’s hardly worth endangering the world trade system. Meanwhile, it encourages Asian countries to sign up for China’s belt and road initiative, trade sphere and Asian bank.

Worse, it makes everyone else adopt the same dog-eat-dog value system. The lesson of history is unambiguous. When the international system becomes a cockpit for economic, trade and national rivalry, armed conflict becomes more likely. Trump and the incompetent ideologues around him may welcome that: nobody else does.

The best response is to show no weakness and stand up to him, as France’s Emmanuel Macron and Germany’s Angela Merkel do. And try to keep what is left of the international system together until eventually Trump is either impeached or voted out of office. But Macron and Merkel have the power of the EU behind them, itself now the world’s best and most powerful custodian of a rules-based international system. Britain is a weakened onlooker – a flyblown lion unable to muster more than a whimper. Our capacity is reduced by Brexit, just as we – and the world – need so much more.

• Will Hutton is an Observer columnist