IT was no exaggeration for President-elect Joe Biden and other members of Congress to describe this week’s storming of the Capitol Building in Washington DC by a mob as “one of the darkest days in recent American history”.

The United States has seen no shortage of violent protests in recent years, but these disgraceful scenes marked a shameful low, since the protestors had been assembled and encouraged by the outgoing President, Donald Trump, specifically to frustrate the result of a democratic election.

On the basis of a pack of lies and wild allegations with no evidential foundation, Mr Trump deliberately tried to undermine a poll he had straightforwardly lost. He told supporters, whom he had summoned to the nation’s capital on the day his opponent’s victory was to be certified, that “we will never concede”, then urged them to march on the seat of government. The ensuing melee has so far cost five people their lives.

Responsibility for this deplorable riot is his. Whether it was organised enough to qualify as “an attempted coup” or “insurrection”, as some have described it, it is clear that there were some who would have liked it to be. The discovery of pipe bombs and Molotov cocktails certainly justify the description of domestic terrorism, and it is not unreasonable to characterise Mr Trump’s role as fomenting sedition. This is not merely behaviour unworthy of his office – something Mr Trump has constantly demonstrated – but grounds for his immediate removal and for criminal investigation.

The alarming lesson is how fragile even previously robust democracies become when political leaders flirt with demagoguery, foster division, and trample over fundamental aspects of the process.

The consent and concession of losing candidates is not just a matter of good grace or basic manners. Nor is it up for debate without credible evidence of irregularities. It is a vital component of the electoral process, without which any vote becomes pointless. Whether Mr Trump’s inability to grasp that is due to mental incapacity or malicious disregard for the consequences, it amounts to an attack on freedom and democracy that imperils the principles on which America claims to be founded.

The fact that Mr Trump’s most fanatical supporters are either actively opposed to those principles, or are previously reasonable people duped by his behaviour into ignoring them, demonstrates the danger of brands of populism that depend on exploiting division.

It is not merely that those stances are extreme, but that they tend to generate extremism in the opposition to them. If one side offers some new affront to the norms of political discourse and conduct, it becomes that much easier for their opponents to justify similarly cavalier action in response.

It is a tendency that – though thankfully not at the same level of blatant criminality – can be seen in all sorts of issues that have tribalised and divided politics in recent years. That has been corrosive and damaging. Social media, with its ability to promote unfounded claims, tendency to trigger hasty, intemperate responses and creation of reinforcing bubbles, has compounded this trend.

Many of the biggest and hardest-fought issues of recent years – including Brexit and independence – could hardly avoid being contentious. But it is evident that the discourse could have been conducted in a more civil fashion, and politics is the worse for the rancour that some have demonstrated.

Some pro-Brexit rhetoric on the Right has verged on xenophobia, while the most extreme resistance to it attempted to ignore a democratic result. The “cancel culture” and identity politics in some parts of the progressive Left have been similarly divisive. Here in Scotland, a minority of those advocating both independence and the Union have fallen into intemperate and intolerant positions.

It is quite all right, and there must be the right, for people to hold strong views on such issues. But events in the US show that actively fostering division and characterising political opponents as ill-intended or treacherous is an extremely dangerous game that encourages and enables those with genuinely malign purposes. Civic life, civil discourse, civilisation itself, all require civility.