What is the matter with us? A US president has been invited to Britain on a state visit, and is coming in June. It is a state courtesy, between one democracy and another, on the occasion of a wartime anniversary. No conceivable purpose is served by 200,000 people coming to London to shout insults at him.

I cannot think what possessed Theresa May to invite Donald Trump in the first place, but she did. State visits are like that. The Queen entertained Romania’s Ceauşescu, Zimbabwe’s Mugabe and Zaire’s Mobutu. Alongside them, Trump might seem an angel. Britain has the deepest of bonds with the US, with ties of history, commerce, culture and education. Washington’s present leadership may embarrass, even outrage, people on both sides of the Atlantic. That is no reason for childish protests against a guest invited in Britain’s name.

Others never see us as we see ourselves, and vice versa. Though I sympathised with Donald Tusk’s remark in February about a “special place in hell” for some Brexiters, I found it curiously offensive. I did not warm to similar remarks about Britain from France’s President Macron and the EU’s Michel Barnier. They merely bolstered Brexiters in their aversion to all things European. In my experience, Americans do not understand boorishness in foreigners. They are by nature a courteous people.

We have arguments with Trump’s America, currently over the treatment of Iran, the handling of trade and the risks supposedly involved in the Huawei contracts. We may disagree on migration and human rights. There are channels and platforms for those arguments. Decisions in a democracy should be reached through relentless debate, not through the lobbying of money or the chants of crowds, any more than through the guns of war.

People yearn to express themselves when they feel the conduits of democracy blocked. From Sudan to Paris to London’s Oxford Circus, the city street has lost none of its potency. It appeals to the most basic social instinct, that of like minds congregating. Politics has long been partly a carnival, whether stimulated by Brexit, global warming or Donald Trump. I prefer the fancy dress of Parliament Square to the teargas of the Arc de Triomphe.But unless there are consequential gains to such action, it is mere self-indulgence at other people’s expense. Those fortunate enough to live in a democracy should not lightly short-circuit its framework institutions.

This may seem a poor moment to preach the virtues of parliamentary government, but such institutions are all that stand between formal democracy and the “strong, rule-breaking leader” we are told more and more people are now wanting, right across the west. There is no evidence that such a leader would be a liberal.

‘Since I regard the re-election of Donald Trump as a palpable threat to world concord, I want nothing that might feed the prejudice of his supporters.’ Photograph: Stefan Rousseau/AFP/Getty Images

Combating climate change requires policies so costly and drastic they will come only by winning arguments. Britain has made strides in distancing itself from coal – subject to bitter street protest at the time – and in reducing its carbon footprint, while the rest of the world’s has doubled. The quantities may be challengeable, but it is in Moscow, Mumbai and Beijing that young people should be stopping traffic.

The operative word is debate. I groaned when Greta Thunberg told how she handled those who disagreed with her on global warming: “I don’t.” There was something equally unsettling in the doom-laden hysteria of last week’s BBC climate change documentary. The politics of fear, like the politics of hate, is designed to suppress reason and play on emotion.

No-platforming never wins debates. Abandoning the debating chamber is always dangerous. It invites reaction. Should the police be equally gentle when the Tommy Robinsons, Nigel Farages or Jordan Petersons summon their forces to Waterloo Bridge?

If there is a crisis of liberalism at present, it lies in its retreat to a middlebrow bunker of confirmation bias. It thus becomes reaction’s useful idiot. Since I regard the re-election of Donald Trump as a palpable threat to world concord, I want nothing that might feed the prejudice of his supporters. Since I want to stay close to the European Union, I want nothing that encourages Brexiters to see that as the foible of a metropolitan elite. Likewise I do not want climate change to be dismissed because young people choose to impose economic sanctions on working Londoners.

John Stuart Mill said: “He who knows only his own side of the case knows little of that.” Climate change does not “threaten the planet” – only the lifestyle of its current occupants. That is serious enough for debate. But direct action is not debate. It is an incitement to unreason.

You are welcome, Mr President.

• Simon Jenkins is a Guardian columnist