Confusing civility with comity is a grave mistake in human or international relations. Yes, the G20 summit did agree on a common communiqué after the leaders’ meeting. Some see this as an achievement or an indication that some normality in international relations between the US and other countries is being restored. The truth is that at no previous G20 meeting did the possibility that there would not be a common statement agreed by all participants occur to anyone.

Rather than seeing agreement as an achievement, it is more accurate to see the content of the communiqué as a confirmation of the breakdown of international order that many have feared since the election of Donald Trump. The president’s behaviour in and around the summit was unsettling to US allies and confirmed the fears of those who believe that his conduct is the greatest threat to American security.

The existence of the G20 as an annual forum arose from a common belief of major nations that there was a global community with common interests in peace, mutual security, prosperity and economic integration and the containment of threats even as there was competition between nations in the security and economic realms. The idea that the US should lead in the development of the international community has been a central tenet of American foreign policy since the end of the second world war.

Since his election, Mr Trump’s rhetoric has rejected the concept of global community, and expressed a strong belief that the US should seek better deals rather than stronger institutions and systems. In the past month and especially after the G20, it has become clear that Mr Trump’s actions will match his rhetoric. The US is now isolated on the question of how to deal with the long run security threat of climate change. It has forced the G20 to back away from previous commitments to rejecting protectionism. And in part because of American attitudes, the G20 was mute on international migration at a time when refugee issues are more serious than at any moment in the past 50 years.

All of this is troubling enough. What many people fear but few are saying is that in the difficult times that come during any term the president’s character will cause him to act dangerously. As biographer Robert Caro has observed, power may or may not corrupt but it always reveals. Mr Trump has yet to experience a period of economic difficulty or any form of international economic crisis. He has not yet had to make a major military decision in time of crisis. Yet his behaviour has been erratic.

The president chose hours before meeting Russian President Vladimir Putin to cast doubt on judgments of the US intelligence community regarding Russia’s interference with the US election. On the brink of the most important set of international meetings of his presidency so far, he put forward the absurd idea that a main discussion item at the G20 involved Hillary Clinton’s campaign manager, making demonstrably false assertions about his role.

It is rare for heads of government to step away from the table during major summits. When it is necessary, their place is normally taken by the foreign minister or another very senior government official. There is no precedent for a head of government’s adult child taking a seat, as was the case when Ivanka Trump took her father’s place at the G20. There is no precedent for good reason. It is insulting to the others present and sends a signal of disempowerment regarding senior officials.

Mr Trump’s pre-summit speech in Poland expressed the sentiment that the primary question of our time was the will of the west to survive. Such a sentiment is inevitably alienating to the vast majority of humanity that does not live in what the president considers to be the west. Manichean rhetoric from presidents is rarely wise. George W Bush’s reference to an “axis of evil” is generally regarded as a serious error not because the nations he referenced were not evil but because his rhetoric drew those adversaries together. Invoking the idea of the west against the rest as the president did is a graver mis-step.

A corporate chief executive whose public behaviour was as erratic as that of Mr Trump would already have been replaced. The standard for democratically elected officials is appropriately different. But one cannot look at the past months and rule out the possibility of even more aberrant behaviour in the future. The president’s cabinet and his political allies in Congress should never forget that the oaths they swore were not to the defence of the president but to the defence of the constitution.

The writer is Charles W Eliot university professor at Harvard and a former US Treasury secretary

Letter in response to this article:

Donald Trump or Tezcatlipoca? / From Cathal Rabbitte, Villars sur Ollon, Switzerland