Though he denies it, Mr. Johnson may well leave Northern Ireland inside the European customs union to get a withdrawal deal. He has few other options, and the international community is calling on him to keep the Good Friday Agreement intact.

Details: Mr. Johnson is weighing a proposal that would put parts of the Northern Ireland economy into an “all-Ireland” zone, presumably subjecting them to European Union rules and standards and in that way preserving the north’s open border with the south.

Quotable: “It’s basically the only off-ramp being offered,” said one expert.

Operation Yellowhammer: Food and medicine shortages, dayslong waits at the border and civil unrest are possible in the event of a no-deal exit from the bloc, according to an internal document Parliament forced the government to release. Read the document for yourself.