Philippine President Rodrigo Duterte (left) shakes hands with President Trump during the opening ceremony of the 31st Association of Southeast Asian Nations Summit in Manila, Philippines on Nov. 13. (Noel Celis/POOL/EPA-EFE/REX/Shutterstock)

Ruth Ben-Ghiat, a professor of Italian studies and history at New York University, is author of the forthcoming book “Strongmen: How They Rise, Why They Succeed, How They Fall.”

Many have sought to diagnose President Trump’s mental health from afar, depicting him as delusional, narcissistic or a madman. Such an approach is understandable, since many of us struggle to make sense of his destructive behavior and attachment to falsehoods that are often of his own making. Yet it’s the history of authoritarianism that provides the best framework for understanding Trump’s words and actions. From Benito Mussolini onward, strongmen have ruled through a combination of seduction and threat, building up protective cults of personality and relentlessly pushing their own versions of reality until they’re in a position to make them state policy.

Far from being lunatics, leaders such as Trump are opportunists and skilled manipulators who may change their ideas on specific policy issues without ever deviating from their main goal: the accumulation and steady expansion of their own power. The one-party state may be mostly a thing of the past, but the authoritarian playbook — and the ways we respond to it — has proved surprisingly durable.

The President has proved to be skilled at strongman strategies and tactics, starting from the one that makes the others possible: forging direct bonds with followers based on loyalty to one’s person rather than to party or principle. Over the past century, rallies and moving images of the leader have been the most effective means of establishing links between autocrats and their supporters. Trump has added Twitter to the mix, enabling him to control the media cycle and manage mass emotions.

For men who care only about political survival (and personal enrichment), creating such leader-follower bonds also offers protection against the institutions that challenge their power. When Trump tells the country “I am the only one that matters,” and plays musical chairs and ritual humiliation games with his cabinet to underscore that everyone else is expendable, he’s not just channeling his inner narcissist or egomaniac. He’s building on a century of proven strongman practices to consolidate his power.

Strongmen are often disrupters and political shapeshifters. Their ability to be many things to many people produces motley crews of followers: small businessmen and clergy, military commanders and racist thugs, billionaires and the poor. Trump has cobbled together a similar coalition. He has also co-opted elites skillfully enough to get into power and govern. The Republicans have now assumed the role of those enablers who throughout history have convinced themselves that the leader will “calm down” once in office (the delusion of the Trump “pivot”), despite evidence to the contrary. After these elites have worked out the terms of their support, they usually back the leader no matter what he says or does, unless a serious economic downturn, coup, or military defeat looms on the horizon.

The GOP has behaved according to plan, sticking with Trump through the allegations of sexual assaults, conflicts of interest and the Russia investigation. Individual politicians may mutter that Trump is volatile and even deranged, but both the president and his enablers have acted in a highly rational and predictable manner. The history of authoritarianism reveals their logic.

The establishment of a culture of threat meant to intimidate opponents and keep allies in their place is the third crucial page of the authoritarian playbook. Unlike democratic leaders, many strongmen have tended to advertise their own personal capacity for violence. It wasn’t just machismo when Mussolini declared himself Italy’s chief criminal in his 1925 speech announcing the onset of dictatorship. The same is true of Rodrigo Duterte when he boasts that he threw an enemy out of a helicopter (taking a page from Augusto Pinochet), or when Mobutu Sese Seko reportedly proclaimed that he fed his enemies to crocodiles.

Trump’s campaign-era declaration that he could stand on Fifth Avenue and shoot someone without losing any followers, which nonplussed many, makes sense within this tradition, as does his persistent choice to sanction agents of violence through his retweets of neo-Nazi and other extremist propaganda. Authoritarians know that such threats, far from harming their popularity, are an integral part of their charismatic and masculine appeal — and are also warnings to critics and enemies that they consider themselves untouchable with respect to the law.

For a man many see as deranged, Trump has been remarkably consistent in his deployment of strongman tactics, as his latest tweetstorm shows. Over the span of a few hours on Wednesday, he called for a boycott of his arch-enemy CNN, retweeted the British right-wing anti-Muslim activist Jayda Fransen (thus legitimizing and publicizing a woman who was convicted for religiously aggravated harassment), puffed up his personality cult by drawing attention to his supposed likability, urged the public and the press to investigate the private life of a citizen (NBC’s Andrew Lack), and commented on the robust stock market.

Such tenaciousness has paid off for Trump, a man who considered running for office years ago but waited to pounce until eight years of Barack Obama’s presidency and progress in gender issues had raised conservative white resentment to the right degree. Our political climate has shifted rapidly since the election; fringe figures have been legitimized and extreme speech and images are now part of our everyday social media diet. Nazis are no longer figures from the past, but our next-door neighbors, and hate crimes have risen since last year. House Speaker Paul D. Ryan (R-Wis.), Vice President Pence and GOP luminaries strain to show obeisance, Pence being especially careful to acknowledge his leader in his tweets and other public communications.

Trump is not a fluke. Nor is he merely an irrational agent of chaos. He’s the American symptom of a cultural and political shift away from liberal democracy that has a long and destructive history. The sooner Americans see his method, rather than his madness, the more effective our opposition to him can be.