Like trauma victims replaying the calamity that befell them while they wonder how much they are to blame, Americans have spent almost three years asking themselves the anguished question that Tim Alberta poses in the stark first sentence of his book: “How did Donald Trump become president of the United States?”

Answering this “riddle”, as Alberta calls it, is like rationalising the fall of man or coming to terms with a cancer diagnosis. Along with WikiLeaks and the KGB, higher powers seem to have been in play. Former White House press secretary Sarah Sanders recently suggested that Trump was God’s personal choice to occupy the Oval Office. Or is he the result of a diabolical jest? Paul Ryan, who decided when he was speaker of the House to collaborate with Trump and educate him about government, entered into what his friends called “a deal with the devil” and came out singed, shredded and eternally shamed.

Perhaps God and Satan colluded to dream Trump up: after all, the US is at once a theocracy and an inferno, the New Jerusalem merged with Sin City. But as Alberta’s chronicle of the past decade makes clear, Trump’s origins are actually less metaphysical. Reconceiving politics as entertainment, selling a personality rather than a policy, he offered to distract a frayed, bewildered society from problems he had no intention of solving.

The man is a leering mockery of the principles he claims to uphold

The Republican party destroyed itself to accommodate him, swallowing its ancient conservative precepts. His current enablers were once his most outraged critics, who kowtowed to facilitate their slimy ascent. Mike Pompeo, now plumply bustling round the globe as secretary of state, accused Trump in 2016 of being “immoral and possessing dictator-like qualities”. Mick Mulvaney, his obsequious chief of staff, called him “a terrible human being”. Mike Pence, whose vice-presidential visage is currently fixed in a rictus of pious adoration, went from telling allies that he “loathed Trump” to admitting, after their first game of golf, that he “beat me like a drum”. Could a whipped cur have been more fawningly grateful?

At his inauguration, Trump surveyed the rusty hinterland of what used to be God’s own country and vowed that “this American carnage stops right here and stops right now”. Since he lies whenever his mouth moves, the opposite was true: the carnage – of constitutional norms, international accords and rules of civilised conduct – was about to begin and for Trump, who says at one point in Alberta’s book: “I fucking love this job!”, it has been a gleeful carnival, an opportunity to foment an almost cosmic chaos. The man is a leering mockery of the principles he claims to uphold: a self-invented monster who supports nativism, a celebrity who rails against the media that fuelled his rise, the marketer of an onomastic global brand who wants to restore isolationism.

Nothing could better prove Trump’s nihilistic lack of convictions than a remark Alberta quotes about Alexandria Ocasio-Cortez, the flame-throwing former bartender elected to Congress in 2018. Last week, Trump began attacking her, along with her sorority of ethnically diverse colleagues who are known as “the squad”, and invited them all to leave the country. Yet when he first saw AOC in eloquent action, he was “starstruck”. “That’s Eva Perón. That’s Evita,” he drooled. “I spotted talent. She’s got a certain talent.” With Trump, the adversarial has an erotic charge, which is no doubt why he offered Stormy Daniels his puffy arse to be paddled with a rolled-up magazine that had his face on its cover.

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Those of us watching this grotesque and increasingly obscene farce from afar should ponder a comment made to Alberta by a despairing American conservative: “Populism is not ideological. It basically says, ‘There’s a parade in the street and I’d better get out there because I’m their leader.’” Trump prevailed because the symmetrical model of the two-party state had failed. Nominally a Republican, he gobbled up the elders and sages in his own ranks; now the Democrats are also split between AOC’s incendiary guerrillas and the cautious moderates led by Nancy Pelosi. There is a sinister synergy between these civil wars and the divisions within both Tory and Labour ranks that have left us without a functioning government or a single-minded opposition.

This week, we will probably be asking our own version of Alberta’s question, preferably with a few added expletives: “How did Boris Johnson become prime minister of the UK?” The reasons are the same – institutional malaise and social alienation, exploited by an unprincipled liar – but in turning from the US to the UK we go from the imperial to the insular, from the apocalyptic to the bathetic, from a dragon-breathed ogre to a bumbling clown.

America has always thrived on purgative violence and will absorb the current carnage and recover from it; detox and rehab are national specialties. But the British carnage is likely to be terminal – an entirely optional exercise in self-harm and self-dismemberment that will leave a butchered rump bleeding in the gutter while BoJo amuses himself by finishing his long-overdue book on Shakespeare.

• American Carnage: On the Front Lines of the Republican Civil War and the Rise of President Trump by Tim Alberta is published by HarperCollins (£23.58). To order a copy go to guardianbookshop.com or call 0330 333 6846. Free UK p&p over £15, online orders only. Phone orders min p&p of £1.99