Trump and outgoing British PM Theresa May. Photo: Dan Kitwood/Getty Images

The weekend leak of a British diplomatic cable savaging President Trump was a minor event by Trump-era standards. That’s in part because it came via the British press on an American holiday weekend, but also because the memo’s author, British ambassador Kim Darroch, didn’t break new ground in assessing the president and his administration. Trump, he wrote, is dim-witted (“you need to make your points simple, even blunt”), susceptible to flattery (“you need to start praising him for something that he’s done recently”), lies constantly, potentially compromised by Russia (“the worst cannot be ruled out”), and generally incompetent (“we don’t really believe this administration is going to become substantially more normal; less dysfunctional; less unpredictable; less faction riven; less diplomatically clumsy and inept”).

We knew all that already. What’s more, we knew that the world (whose diplomatic strategy with the 45th president mixes delicate flattery with low-grade bribes ) knew it already. Even many members of Trump’s own administration know this. Yet Trump’s reaction to the story indicates that he regards it as especially offensive and even dangerous.

Trump told reporters, “The ambassador has not served the U.K. well. I can tell you that. We’re not big fans of that man … So, I can understand it and I can say things about him as well, but I won’t bother.” Later he tweeted out a similar message:

....thought of within the U.S. We will no longer deal with him. The good news for the wonderful United Kingdom is that they will soon have a new Prime Minister. While I thoroughly enjoyed the magnificent State Visit last month, it was the Queen who I was most impressed with! — Donald J. Trump (@realDonaldTrump) July 8, 2019

Darroch has now resigned, an unusual move reflecting the pressure Trump had placed on the British government. It’s notable that rather than lashing out with his customary viciousness, Trump is attempting to isolate the issue to the idiosyncratic views of a single, “wacky” ambassador representing a government that will soon be replaced. Meanwhile, Trump insists he maintains good relations with the royal family and the (unnamed) next British prime minister.

The president’s unusual restraint may reflect two realities that Democrats ought to bear in mind. A pollster once told me that one of the few foreign-policy facts that registers in the minds of the electorate is the country’s standing with its allies; a rift with friendly nations actually bothers lots of people. A Pew survey from a couple years ago found that 59 percent of Americans believe their country “should take into account the interests of its allies even if it means making compromises with them,” while only 36 percent agreed that it “should follow its own national interests even when its allies strongly disagree.” Trump’s notion of “America first” does not command popular support, and hostility toward a country like Great Britain can actually hurt Trump.

Second, the memo validates questions about Trump’s fitness for office. In a recent poll, 65 percent of the public agreed Trump has “acted in a way that’s unpresidential.” Of course, some of that 65 percent approve of Trump’s overall performance anyway — they’re Republicans, after all — but the size of this number shows just how many Americans find his conduct distasteful or worse.

The case against Trump contains so many damning particulars, it is difficult to narrow it down. But the existence of a written memo confirming the view of a close American ally that Trump is utterly unfit for office might qualify as one of those facts the opposition should bring to the public’s attention.