Disagreements over the terms of Britain’s departure from the EU are obscuring the daunting international challenges that await the country once it supposedly shakes off Europe’s shackles. It’s a tough, unforgiving world out there, and 2019 is shaping up to be an even bigger rough-house than usual. By jettisoning a pivotal alliance, Britain no-mates is seriously weakening its capacity to manage these looming threats.

The spectre of Donald Trump lies at the heart of ominous turbulence on the global horizon. Nearly halfway through his term, the 45th US president is helping to create a world where old rules don’t apply and long-held assumptions, such as Britain’s claim to a “special relationship” with Washington, are an anachronistic embarrassment.

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Trump’s is an anarchic realm, dangerous, delusional and chaotic – comparable to a dysfunctional Florida theme park – on which a category five hurricane is bearing down. It is characterised by structural vandalism, and fuelled by self-interest, insults and lies. Trump’s ignorant, confrontational persona informs concerted US efforts to overturn or bypass the rules-based international order – he regularly attacks and undermines the United Nations, the European Union, Nato, the international criminal court, the international court of justice, the World Trade Organisation and efforts to address climate change. It encourages bad behaviour everywhere.

To survive on its own in a world full of hazards, Britain is relying on the familiar frameworks, multilateral institutions, laws, regulations, diplomatic conventions and commercial codes that have governed state-to-state relations since 1945. But it is exactly this consensual rulebook that Trump is recklessly tearing up.

If there is to be a US trade deal with supplicant Britain, for example, it will be on Trump’s onerous “America first” terms. If the whim takes him, his punitive tariff wars will intensify, regardless of their impact on struggling partners. Britain may cast itself as a 21st-century champion of free trade and international engagement, but it is Trump’s protectionist, isolationist and nationalist tropes that are trending worldwide.

Facebook Twitter Pinterest ‘Vladimir Putin’s covert hooliganism extends from the Barents Sea to the Sea of Azov.’ Photograph: Alexei Druzhinin/Sputnik/Reuters

In Trump’s fearful world, the night is full of terrors, and that’s especially true of 2019. The president needs a scary distraction from the deep legal and political trouble he is in at home, especially as he desires a second term. This distraction would preferably come from overseas – for in Trump’s world, foreigners are defined not as friends but as potential foes. Deliberately intensifying the US confrontation with Iran is one deflective possibility. Trump has worked hard to provoke Tehran, reneging on the 2015 nuclear deal and imposing new sanctions over British objections. The Israelis and Saudis would be up for a fight. But for Britain, it would be an unpropitious start to post-Brexit global outreach. At the same time, Trump’s hostility to existing military deployments overseas presage problems on other key policy fronts. A prime example is his rash decision to ignore UK advice and abandon the fight against Islamic State in Syria. US troop withdrawals from Afghanistan, where Britain recently increased troop numbers, may follow. James Mattis, the defence secretary, could not stop him, and resigned in protest. After Brexit, belittled Britain will have less say than ever.

Russia is a leading beneficiary of Trump’s contempt for western solidarity and shared values – and a big problem for Britain no-mates. Vladimir Putin poisoned at will in Salisbury, subverted the Brexit vote, and regularly violates British sea, air and cyberspace. His covert hooliganism extends from the Barents Sea to the Sea of Azov. Russian “malign activity” in 2019 will underscore the reality that, in or out, Britain’s external defence and security remain intimately linked to Europe’s.

Then there are EU allies to consider. Are they now to be forgotten? Angela Merkel, a European standard-bearer reviled by Trump and German xenophobes alike, is on her way out. France’s unpopular president, Emmanuel Macron, is besieged. If the Franco-German centre cannot hold, rightwing populist-nationalists from Italy to Poland will profit. The threats to Europe’s democratic order are real and pressing. This is Britain’s fight, too, as history shows.

So forget the Irish backstop for a minute. Forget the price of fish. Who will stop Putin and the onwards march of Europe’s hard-right? Not Trump. He welcomes anything that weakens the EU. If or when such crisis points arrive, even the most intransigent Eurosceptic may finally grasp the inescapability of Britain’s European ties and the true worth of European unity.

The post-Brexit challenge posed by China is of a different order: as much moral as economic. The Chinese president, Xi Jinping, is running one of the most repressive regimes on Earth, under whose grim auspices Muslims in Xinjiang, Buddhists in Lhasa, “house church” Christians in Beijing, pro-democracy activists in Hong Kong, and independent media, academics and writers suffer hugely. Trump does not give a hoot about human rights in China or anywhere else. But the British traditionally do. Does Britain really want to become further entangled with a communist dictatorship whose internal repression is matched by an aggressively expansionist, illiberal world view? As the head of MI6 noted recently, there are also grave security risks especially if, as in Britain, Chinese companies are investing in nuclear power and telecoms infrastructure. So, bottom line: how much would Liam Fox and friends give up to sweeten a deal with Beijing? They should remember, while counting pennies and yuan, that values matter more than VAT.

Facebook Twitter Pinterest ‘Does Britain really want to cosy up to Recep Tayyip Erdoğan?’ Photograph: Erçin Top/Anadolu Agency/Getty Images

Trump’s second-rank friends are an equally unappealing bunch. Saudi Arabia is a player these days, economically and politically. Yet British gun-running to Riyadh has always been objectionable, ethically speaking. Dodgy deals of that type will be even more insupportable in future, now we know the Saudi crown prince counts assassins among his subordinates. Turkey is another big market – and no friend to Brussels. But does Britain really want to cosy up to Recep Tayyip Erdoğan, whose idea of a good day at the office is killing Kurds and locking up a newspaper editor or two? Or take emerging Brazil. A lot of jolly juicy opportunities there, as Boris Johnson might say. Except Brazil recently elected a hard-right president who plans to burn down the Amazon for cash. This does not sit well with Britain’s climate change commitments.

If push comes to shove, it’s even suggested Britain could revive old Commonwealth ties to sustain its venture into the unknown. But who in the modern Commonwealth needs Britain? India, for example, has moved on since the days of empire. In 2019, its GDP will overtake Britain’s. Any deal with Delhi will be costly. And it may not be long before belittled Britain forfeits its permanent seat on the UN security council, too.

Old roles are reversed. The balance of power shifts. In Trump’s world, nobody respects weaklings and has-beens, especially when they used to run the show (or thought they did). Viewed from all those places formerly coloured pink in the atlas, Brexit increasingly looks like payback time. In spurning its European home, self-destructive Britain casts itself unready and unloved into a world of pain.

• Simon Tisdall is a foreign affairs commentator