LONDON — Europe, the soil on which Fascism took root, is watching the rise of Donald Trump with dismay. Contempt for the excesses of America is a European reflex, but when the United States seems tempted by a latter-day Mussolini, smugness in London, Paris and Berlin gives way to alarm. Europe knows that democracies can collapse.

It’s not just that Trump retweets to his six million followers a quote attributed to Mussolini: “It is better to live one day as a lion than 100 years as a sheep.” It’s not just that Trump refuses to condemn David Duke, the former grand wizard of the Ku Klux Klan, who has expressed support for him. It’s not just that violence is woven into Trump’s language as indelibly as the snarl woven into his features — the talk of shooting somebody or punching a protester in the face, the insulting of the disabled, the macho mockery of women, the anti-Muslim and anti-Mexican tirades. It’s not just that he could become Silvio Berlusconi with nukes.

It’s the echoes, now unmistakable, of times when the skies darkened. Europe knows how democracies collapse, after lost wars, in times of fear and anger and economic hardship, when the pouting demagogue appears with his pageantry and promises. America’s Weimar-lite democratic dysfunction is plain to see. A corrupted polity tends toward collapse.

Trump is telling people something is rotten in the state of America. The message resonates because the rot is there.