How quickly, in a crisis, the unthinkable can become reality. If it might have seemed fanciful a week ago that a US president could incite insurrection or face a second impeachment in a single term, so, too, would the prospect of social media platforms barring the “leader of the free world”. The bans by Twitter, Facebook and Instagram on President Donald Trump raise profound issues — of freedom of speech, and the precedents they may set for less free societies. While the “permanent” nature of Twitter’s suspension is questionable, however, not acting would have created even bigger risks.

In the exceptional circumstances of America today, platform companies were right to suspend Mr Trump’s access at least until the end of his presidential term. The president has glorified violence and egged on a challenge to US institutions that left five dead. Critics are right to say the move came cynically late. The president has repeatedly flouted the platforms’ user rules. Had the platforms acted earlier to remove offending presidential posts selectively, the need for tougher actions might have been averted.

Police had good reason for concern, moreover, that the president’s supporters were using both mainstream and more niche platforms to plot further violence. That justifies moves by Apple, Google and Amazon to restrict access to Parler, the “alt-tech” Twitter alternative beloved of the radical right.

These are, without doubt, complex ethical issues. German chancellor Angela Merkel criticised Twitter’s indefinite suspension of Mr Trump as a breach of the “fundamental right to free speech”. Alexei Navalny, the Russian anti-corruption blogger, said it could be “exploited by the enemies of freedom of speech around the world”.

Yet America’s ability to promote democracy and political freedoms elsewhere collapses if it cannot defend its own. While some question why US enemies retain Twitter access while Mr Trump is barred, the president has unique power to undermine American democracy, should he so choose. It is misleading to suggest Mr Trump has been “silenced” when he retains access to the powerful US presidential pulpit.

Free speech, moreover, cannot be wholly untrammelled. Liberals should be wary of their own arguments being misused to undermine what they believe in. Constraints are legitimate on hate speech and online incitement. Though the cultural context is very different, Ms Merkel notes the US would be better to follow Germany in passing laws restricting such behaviour than leaving it to social media platforms to devise and police their own rules.

Such legal restraints in the US might run into First Amendment problems. Yet recent days highlight above all the need for debate on the limits of American free speech and the power of the tech companies. Clearer regulation must be a priority for the incoming Biden administration and for Congress.

That may not mean repealing Section 230 of the Communications Decency Act, which gives internet companies immunity for user-generated content on their sites. But it should at least be reformed, with exemptions extended to cover, for example, incitement to violence or terrorist propaganda. A more effective redress mechanism is needed.

What cannot be overlooked, either, is the responsibility of conventional TV outlets such as Rupert Murdoch’s Fox News, which has enabled Mr Trump for too long, and according to one study has been more influential in spreading false beliefs than social media. The UK, which has given approval for a Murdoch-owned “opinionated” news channel, should pay careful heed.