Trump has changed the rules of engagement. It's time to wise up Simon Tisdall

It's not only about Donald Trump. It's not just about saving Ukraine, or defeating Russia, or how to boost Europe's security, or what to do about an America gone rogue. It's about a world turned upside down - a dark, fretful, more dangerous place where treaties and laws are no longer respected, alliances are broken, trust is fungible, principles are negotiable and morality is a dirty word.

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It's an ugly, disordered world of raw power, brute force, selfish arrogance, dodgy deals and brazen lies. It's been coming for a while; the US president is its noisy harbinger.

Take the issues one at a time. Trump is a toxic symptom of the wider malaise. For sure, he is an extraordinarily malign, unfeeling and irresponsible man. He cares nothing for the people he leads, seeing them merely as an audience for his vulgar showmanship.

His undeserved humiliation of Ukraine's valiant leader, Volodymyr Zelenskyy, was, he crowed, "great television". As president, Trump wields enormous power and influence. But Potus is not omnipotent.

America's vanquished Democrats are slowly finding their voice. Connecticut senator Chris Murphy shows how it should be done. When Trump tried to blame diversity hiring policies for January's deadly Potomac midair collision, Murphy hit back fiercely.

"Everybody in this country should be outraged that Donald Trump is standing up on that podium and lying to you - deliberately lying to you," Murphy fumed. Trump was at it again when he mugged Zelenskyy last week. But it is not passing unchallenged. Street protests followed.

A campaign gathers pace to block Trump's planned UK state visit. Opinion polls show growing opposition.

Resisting Trump is what our leaders must do. The world's most admired democracy is held hostage by a far-right clique of thugs and chancers. Its leader calls himself "king" and talks of a presidency for life. Elon Musk and Steve Bannon raise stiff-armed salutes.