The failure of the G7 meeting in Charlevoix, Quebec, marks a watershed for the 21st-century democracies. It is the moment when Donald Trump’s disruption of the international order moved from annoying threat to damaging reality. Mr Trump went to Quebec only under protest. He made no effort to compromise on his tariff war. He arrived late for meetings, chided the other leaders and left early. He snubbed the final communique. He tweeted insults to his Canadian hosts from his plane as he headed off. By the time his North Korean summit ends later this week, Mr Trump may be on chummier terms with another authoritarian dictator than with America’s democratic allies.

Play Video 1:54 'We're like the piggy bank that everybody's robbing' says Donald Trump at G7 – video

The immediate upshot is bad enough. The US refused to sign a G7 communique that was too weak, especially on climate change, but that should have been supported. Mr Trump’s steel and aluminium tariffs were the flashpoint; but the G7 countries actually have much in common on trade, not least on forced technology transfers to China. The communique also challenged Russia’s destabilisation tactics, said Iran’s nuclear programme must remain wholly peaceful and contained declarations on gender equality and fair tax regimes. Mr Trump has not signed off on these either. But the longer-term damage of this division could be even worse.

Some will argue that this was merely another bout of Trumpian theatrics, inseparable from his boundless narcissism and essential for fuelling his supporters’ anger in election year, and that the underlying substance of the US’s relationship with its fellow liberal democracies remains resilient enough to survive. This has been the default position of most of America’s allies ever since Mr Trump became president. They are doing their self-sacrificing best to keep him in check in the hope that he will be a passing phenomenon. This view should not be dismissed. But it has become too naive.

The truth may now be more serious. The US is the cornerstone of the post-1945 international order. If Mr Trump wishes to remove that cornerstone everything else is threatened. That has not yet happened. The thankless task of doing what can be done to mitigate Trumpian disruption must continue. But a fissure is growing. And it got wider in Quebec.

Without the US the other G7 nations lack as much clout to make the system work. Only France of the four European nations at the summit has anything approaching a strong government. The EU remains feeble. But the rest of the G7 must try. If not, Europe, Canada and Japan risk becoming standing invitations to humiliation by Russian disruption, Chinese strategic authoritarianism and Trumpian nativism. This puts the liberal value system of democracy, peace, trade, liberty and the rule of law at risk. It threatens the ability of the liberal democracies to stand up to threats. Not for the first time.

The G7 is an imperfect mechanism for holding back threats to rules and order. It was found wanting during the financial crisis. Its relationship with the G20 of rising, as well as established, economic powers is intermittent. It can appear portentous and smug. But, as with other deeply imperfect and insufficiently effective international bodies, it is a mistake to make the best the enemy of the good.

This is a particularly powerful lesson for Brexit Britain. The Quebec G7 is a reminder to Britain of many things: that trade is a multilateral not a bilateral issue, that the US is not necessarily our reliable friend and that the great nations of Europe, of which Britain is one, share the same values and interests. These lessons have rarely been more in need of restating.

After the fall of France in June 1940, David Low published a famous cartoon. It showed a solitary British soldier, standing on a rock in stormy seas, with his fist and face raised to the heavens. The caption stated simply: “Very well, Alone.” Now, 78 years later and in peacetime, Britain faces a different moment of decision. Shockingly, today’s threat comes from our wartime ally, the US, in the shape of President Trump. If Low was alive today, he might draw a similar cartoon. This time, though, there would be a Frenchman, a German and others on the rock with the Briton, and the caption would read: “Very well, Together.”