How to deal, psychologically speaking, with Boris Johnson and Donald Trump? For obvious reasons, many of us have spent the past several years telling ourselves that these (insert your own insult here) politicians are outrageous anomalies, mere boils on the otherwise unblemished face of the body politic. Of course boils are nasty, their suppurating eruptions highly unpleasant for anyone who happens to be in the vicinity. We all know this. But we take comfort in the fact that they may also be treated. The condition, ultimately, is curable. The state – I won’t say deep state – will carefully apply its very own brand of topical antibiotics, and the inflammation will eventually disappear, smoothness restored at last.

But is this really true? The writer Ferdinand Mount doesn’t believe that Johnson and Trump are as exceptional as we might like to imagine – we must, Mount instructs, abandon the comforting illusions of historicism, the idea that liberal democracy is inevitably here to stay – and in his brilliant new book, he sets out to put them in context, looking for traces of their tactics and their effects in, among other strong men of history, Julius Caesar, Oliver Cromwell, Napoleon Bonaparte, António de Oliveira Salazar, and General de Gaulle. The result is a kind of field guide to Caesarism; think of it as I-Spy an Autocrat. Mount uses his examples as a means of telling us what to look out for, from propaganda and wall-to-wall lies to a certain (weird) kind of charisma; from careful timing (the authoritarian bides his time) to national susceptibility (peak grievance is his moment). More encouragingly, he also examines how Caesars are unmade, a section that includes accounts of events as disparate as Catiline’s attempt to seize control of the Roman state in 63BC, Indira Gandhi’s imposition of a state of emergency in India in 1975, and, yes, both the toppling of Boris Johnson by Conservative MPs and the march on the Capitol after Donald Trump lost the American election to Joe Biden in 2020.

But what I’ve written does it no justice. Big Caesars and Little Caesars is a hard book to describe for the simple reason that, while not particularly long, it is unfathomably capacious. Mount, learned to the pink tips of his ears, knows so much, and what he didn’t before, he has found out, making good use of recent books such as Tom Bower’s biography of Johnson and Luke Mogelson’s hour-by-hour account of the Capitol riots. It is a bit dazzling, the way he dances around. Here is Cromwell, smashing up parliament and seizing the mace (“Come, come, I will put an end to your prating”), and here is Napoleon, conquering half of Italy in the name of republican liberty (“What I have done so far is nothing”); here is Julius Caesar about to cross the Rubicon, and here is De Gaulle denying he had anything to do with the insurrection in Algeria. Reading it, I learned a lot, some of which I can already hear myself using. “But don’t you know that a line can be traced from Trump all the way back through Ronald Reagan to Calvin Coolidge, and even to Andrew Jackson?” I will ask next time I meet a complacent American.

And yet it all slips down so easily, Mount’s considerable journalistic skills deployed here in the cause of concision, the pricking of pomposity and, sometimes, his own outrage (it’s fair to say that the author, who ran Margaret Thatcher’s Number 10 policy unit for two years in the early 80s, is as appalled as anyone by the antics of Johnson – perhaps for that very reason). He’s especially good on Johnson, a type doubtless familiar to him (both are old Etonians). His delineation of the former PM’s loneliness, laziness and opportunism is expert, the prefect morphing into a shrink, and retrospectively terrifying (things could have been even worse). “We have in our own time a recent example of a narcissistic soliloquy not unlike Caesar’s, if on a Lilliputian scale,” he says, damning Johnson twice over as he compares the emperor’s cover up of his activities during the civil war in Rome with the former prime minister’s decision in 2016 to announce himself in favour of leaving the EU. For Mount, Johnson’s years at No 10, during which he did not engage with any leader in Europe save for Hungary’s Viktor Orbán (another small-to-middling Caesar), represent, for him, a period of fully sociopathic diplomacy. “The self-styled Spartans of the European Research Group had been calling the shots out of all proportion to their numbers – at least Leonidas had 300 men with him at Thermopylae,” he writes, with characteristic wit (the Greek king died in that famous battle).

Ah yes, his wit. Mount is beautifully wry in this book, on top of everything else. “I can’t think of anything quite so shocking being said in public since Lytton Strachey used the word ‘semen’ in front of the Miss Stephenses in their Bloomsbury drawing room in 1907,” he tells us of the moment when Jeremy Warner, who writes about economics in the Daily Telegraph, finally broke ranks to admit that “project fear” had been right all along about Brexit and its consequences (Warner was emboldened to break the omertà by Johnson’s fall). However, there is a paradox here. Such drollery, far from being reassuring, only has the effect of making the reader take him the more seriously – or at least, this is how it worked on me. Mount insists that he is not pessimistic, and he ends his book with a beautiful and rather extraordinary hymn of praise to parliament and all its necessary “mummery”. Nevertheless, I am taking Big Caesars and Little Caesars as yet another portent. If he is alarmed – and what is his book if not a loudly clanging bell? – then I think the rest of us should be, too. Johnson has gone (we think). But keep your eyes peeled. Doubtless another one just like him is already waiting in the wings.

• Big Caesars and Little Caesars: How They Rise and How They Fall – from Julius Caesar to Boris Johnson by Ferdinand Mount is published by Bloomsbury (£20). To support the Guardian and Observer order your copy at guardianbookshop.com. Delivery charges may apply