The apparent decision of British voters to hang their parliament holds a warning for President Trump. Not to mention the catastrophe it represents for Britain. After all, the heroic vote for independence, made by Britain a year ago, turned out to be a precursor of Mr. Trump’s own victory here. One doesn’t want to get into a situation where the reverse is true.

Not that the vote today in Britain was a vote against Brexit. Neither of the two leading parties campaigned to reverse the decision a year ago in favor of an independent Britain. Rather, the vote today strikes us as a warning to Mr. Trump against failing to deliver on for what the voters asked. In Britain, that was an unambiguous decision to be an independent country.

Yet after the vote for Brexit, the backsliding began almost immediately. It started with the decision to bring in as prime minister a woman who, in Theresa May, had campaigned against Brexit. Less than a year ago, the lady was all for staying in Europe. Come the vote, Mrs. May announced she would bow to the voters, but all too often it looked as if her heart wasn’t in it or her judgment was lacking.

Mrs. May dallied before triggering Article 50, leaving an opening for a lawsuit that threw the matter to Parliament. She won that test, but by then the better part of a year was gone, and the country has been consumed with a debate over whether to go for a hard — meaning complete — exit from Europe or a soft one, meaning some sort of compromise.

The New York Sun favors a hard, complete, unambiguous, permanent, irreversible exit from Europe and a stronger relationship with the pro-liberty democracies. In Parliament, in any event, Mrs. May wanted a stronger margin of support to give her a stronger hand in dickering with Europe. In the odd world of Britain, that meant a stronger hand so she could compromise with Brussels.

Mrs. May, meantime, isn’t the only one who dallied. Mr. Trump was the only pro-Brexit politician in America, which, in our view, is one — only one, but one — reason he won the 2016 election. He tapped into the same sentiments that animated Brexit and even campaigned, here, with the leader of the United Kingdom Independence Party, Nigel Farage.

In January, Mrs. May made a fine visit to America, and President Trump declared that a “a free and independent Britain is a blessing to the world.” Yet hopes for a fast start on a British-American trade deal faded quickly, as Mr. Trump was entangled in scandal by those bent on resisting his election. Legislation is dragging in Congress for want of strategic vision and leadership.

That left Mrs. May with a weaker hand in Britain and in Europe. And Mr. Trump without a template for other bilateral relationships at the very moment he is trying to break free of the octopus-like multilateral relationships of all kinds, from the Paris climate accord to the United Nations and the Iran deal. Mrs. May floundered on domestic issues, as well.

We like the way it was put in the Sun yesterday by our occasional contributor, Canada’s Stephen MacLean, of the Disraeli-Macdonald Institute, who stressed Britain’s moral duty in respect of Brexit.

Today’s result — if the exit poll holds up — will be a breath-taking defeat in an election that she called with the idea of sharply expanding her majority in Parliament. It looks like the Tories will have less than a majority, that Labor will have an expanded minority but well short of the Tories, that the Scottish National Party has been cut back, and that the center-left liberals have gained seats.

We wouldn’t put ourselves forward as experts on Britain, but it’s hard to see the logic in how Mrs. May stays on as premier. Better to find someone who really believes in Brexit and can try to salvage British independence from the wreckage. The gains by Britain’s Labor Party, led by a leftist in the ilk of Senator Bernie Sanders, is another result suggesting Mr. Trump needs to start delivering on the economy and foreign policy, the key promises of his own winning campaign.