Donald Trump’s ungainly back-and-forth on immigration has a parallel in Britain, which is struggling to make sense of its own impetuous resolution to take control of its borders. Indeed, if Britain after the Brexit referendum is anything to go by, a Trump presidency would be dominated by zigzagging: sometimes to dilute past promises, sometimes to double down. In the terrifying event that Trump actually became president, you’d hear supporters grumbling bitterly about treachery — even as critics wondered furiously why impractical campaign pronouncements were so seductive for so long.

More than two months after their vote to leave the European Union, the British are no closer to understanding what they have done. Theresa May, the sensible prime minister, assures the public firmly that “Brexit means Brexit,” much as parents tell their children that bedtime means bedtime. But May stoutly refuses to specify what she means by this. On Wednesday she summoned her cabinet for a special off-site meeting, and her spokeswoman declared afterward that Britain wanted the right to curb migration from Europe — but also a “positive outcome” on trade. What if these goals are incompatible? The prime minister does not say.

May’s problem is that the Brexit referendum, like the Trump phenomenon, was largely an expression of hostility to immigrants. A survey of more than 12,000 voters on the day of the referendum found that the most common reason to support Brexit was an urge to assert sovereignty; second came the desire to control national borders. Although May herself was a quiet Brexit opponent, she understands the public’s view on immigration. And that makes it almost impossible to envisage a “positive outcome” for trade.

In the European Union, the principle of free movement is nearly as sacrosanct as the Commerce Clause in the United States. This is not necessary or logical: For members of the euro zone, there is an argument that a common currency requires a single labor market; but for E.U. countries outside the euro, you could imagine a union with borders and passports. Yet Norway, which is not in the E.U. but is a member of the E.U. single market, is required to accept unlimited numbers of E.U. migrants as a condition of its trade access. Switzerland, another E.U. outsider that enjoys many of the advantages of single-market membership, wants to control its border but faces similar constraints.

Since the Norwegian and Swiss trade models are incompatible with the Brexit mandate on migration, British politicians have become instant experts on Canada’s free-trade deal with the E.U. But this has its own problems. Aside from the fact that its ratification is uncertain, Canada’s deal mostly excludes services, which account for a hefty four-fifths of British GDP. A copy-Canada deal would not help Britain’s chief exports: financial services, legal advice, architecture and so on.

Faced with no attractive way forward, May is shuffling sideways. She has cannily appointed three leading Brexiteers to cabinet positions dealing with Europe and invited them to propose a solution to their mess. The Brexiteers dislike each other and are generally clueless, so progress has been glacial — the more so because the British civil service has yet to recruit the trade experts and lawyers necessary to make Brexit happen. If the 3.5 million other Europeans in Britain all suddenly applied for permanent residence, it would take the existing immigration staff 140 years to deal with the onslaught.

Before the Brexit referendum, Britain’s Trump-like fantasists assured voters that it would be easy to negotiate a great trade deal with Europe. Now that reality is dawning, there is delicious speculation that Britain might postpone Brexit — perhaps indefinitely. Before formal negotiations begin, Britain must trigger Article 50 of the Treaty on European Union, and May has always said that this won’t happen before January. But the combination of confusion in London and elections next year in France and Germany makes further can-kicking quite plausible. Much as Turkey has been negotiating E.U. accession for years without joining, Britain could proclaim that Brexit means Brexit but not actually leave.

Of course, the prime minister denies this. “There’s no second referendum; no attempts to sort of stay in the E.U. by the back door,” she reiterated Wednesday. But the embarrassing fact is that more than three-quarters of her cabinet opposed Brexit, and for excellent reasons. More than 40 percent of Britain’s exports go to the E.U. The country benefits from collaboration with its neighbors on everything from scientific research to counter-terrorism.

However things turn out for Britain, the lesson for Americans is stark: Refuse to be seduced by campaign pledges that could not possibly be implemented without damaging the nation. Whatever the glib talk of post-truth politics, the truth still matters when it comes to governing.