President Trump participates in a working session during the Group of Seven summit in Charlevoix, Quebec, Canada, on Friday. (Saul Loeb/AFP/Getty Images)

THE UNITED STATES, Britain, Germany, France, Italy, Canada and Japan formed the Group of Seven as a sort of regular conclave among the world’s leading capitalist democracies about 40 years ago, during the Cold War. Its initial focus was economic, to prevent precisely the sorts of trade issues that President Trump is now inflaming from growing into political splits that could endanger solidarity in the face of what was then the Soviet threat. In the post-Cold War world, the G-7 morphed into an institution expressive of the West’s determination to perpetuate the values — free markets, representative government, the rule of law — that its members considered to have been vindicated by the Soviet empire’s collapse.

It follows that Mr. Trump’s decision to sow ferment within the G-7 — to antagonize America’s closest allies — is more than a childish tantrum or a play for attention. It is an undermining of those values that the G-7 was meant to safeguard. Not only the allies but Americans, too, must come to grips with the fact that the ostensible leader of the free world may not really believe either in the free world or in leading it.

Mr. Trump joined the other G-7 leaders at a summit in Canada on his way to meet the dictator of nuclear-armed North Korea. It is a moment when a rational U.S. leader would recognize the benefit of having allies in his corner. Every diplomatic problem is easier to handle with a little help from your friends, and the problem of getting Kim Jong Un to denuclearize verifiably is about as hard as they come.

Yet Mr. Trump chose precisely this moment to browbeat other democracies about what are, in the greater scheme of things, petty trade issues. Other presidents have understood that the United States has gained, disproportionately, from a system in which it helps keep the peace without keeping crabbed accounts on its national ledger. Mr. Trump sees his duty as the issuance of ill-defined demands for “fairness” on countries that, if they so chose, could tick off their own long lists of U.S. economic practices that do not please every single one of their domestic constituencies. Mr. Trump’s bone of contention, Canada’s admittedly unwise and protectionist dairy “supply management” policy, illustrates the point, since the United States also has elaborate programs to prop up its dairy farmers, to protect sugar planters, and so on.

Meanwhile, Mr. Trump remains indefatigably conciliatory toward Russia, which he proposed to readmit to the G-7. This is consistent with his admiration for strongman rule and with his transactional, amoral notion of relations with other countries, but it would contradict punishments that Western countries, including the United States, have just imposed for Russia’s unconscionable assassination attempt in Britain. And it would reward Moscow at a time when its interference in the 2016 election remains unresolved.

“We have a world to run” was Mr. Trump’s justification — which will come as a surprise to countries that have not agreed to be “run” by him, or Russia, or the other G-7 members, for that matter. If it is the United States’ portion, in partnership with other peer nations, to lead, that imposes a responsibility to do so in more than its own narrow self-interest.