The election message was a pledge to “Get Brexit Done”. Repeated on posters, adverts, aprons, T-shirts. Printed wherever there was a space.
The challenge now is to fulfill that promise, or perhaps even to define it. What counts as Brexit? Is it simply leaving the European Union, come what may, on the last day of the year, or is it about achieving a comprehensive trade deal?
What sort of relationship does Mr Johnson want with Europe, and what sort of compromises is he prepared to make?
You might have thought those questions would have been answered during the campaign but – emphatically – they weren’t.
Mr Johnson, for instance, insists there will be no checks or tariffs on goods crossing the Irish Sea from Britain to Northern Ireland.
Many think that promise is simply impossible to keep, even if a trade deal is signed.
If Britain were to leave without a trade deal, it is even harder to work out how Mr Johnson can live up to his promise.
And that is a prospect that many take realistically in Brussels.
At the moment, Britain doesn’t have a structure for negotiating a trade deal – no specialised teams up and running yet, no guarantee of who will be in charge of each different bit, or how power will be devolved and then used.
The EU, on the other hand, has a department ready and willing, under the auspices of Michel Barnier, the highly experienced French politician who led the first phase of Brexit negotiations.
What’s more, the European Commissioner in charge of trade is Ireland’s Phil Hogan, who brings his own deep knowledge of cross-border trade.
Stacked up behind them is the machine of the European Commission which, love it or hate it, has huge experience in framing trade deals.
The UK is up against a formidable team of negotiators, and Mr Barnier has already talked of it being unrealistic to sign a fully-formed deal by the end of 2020.
But there are plenty of others across Europe who think a deal of some kind can be reached, even if it isn’t the sort of deal that many really want.
Instead, as I first reported on Sky News a couple of months ago, the momentum is swinging towards a “pared-down” sort of agreement that would allow a basic flow of trade but would, at least in its first iteration, inevitably leave holes.
It would be the bare minimum required to establish some sort of trade agreement, but comes with a couple of big problems.
The first thing is whether it can be achieved in the time left. Despite the cheery confidence of some leaders arriving at the European Summit this week (“you can do an awful lot in 11 months,” smiled Luxembourg Prime Minister Xavier Bettel), no deal of this sort has ever been signed off in such a short period of time.
The flip side is that a deal between the UK and the EU should be a lot more straightforward than one between, say, Europe and Canada.
You start from the position where both sides already share the same regulatory structures, and neither wants to diverge hugely from them.
You also have acceptance, on both sides, that it’s in nobody’s interest to rip-up existing agreements on, for instance, airplane travel or security information.
Multiple well-placed sources have told me that these things will be fiddly, but achievable, in some form, by the end of the year.
The sticking point will be something called Level Playing Field regulations – in other words, the requirement that the UK should align itself to regulations laid down by the EU.
The Europeans don’t want to back down, fearing that the UK will reduce its standards in order to obtain a competitive advantage.
That flexibility, though, is precisely the sort of thing that Brexiteers have desired from day one.
Compromise won’t come easily, but one model being considered is the idea of drawing a line in the sand now – and having both sides promising not to regress from present-day regulatory standards.
The British are enthusiastic about this; the Europeans much less so. That may be the key obstacle to getting even a “pared-down” agreement.
But a fully-fledged trade deal? Talk about that, and that’s when people start to shrug their shoulders, and whistle through their teeth.
These are complex deals that require huge scrutiny in every member state and, right now, few believe such an agreement can be reached by the end of the year.
The EU is already suggesting that it would like an extension, so talks could stretch on into 2021, or even 2022. Mr Johnson says that won’t happen; that the UK will leave, come what may, at the end of 2020.
But it’s worth noting that, even if the prime minister changed his mind and decided to extend trade talks beyond the end of next year, it can’t – in theory – be done at the last minute.
A request for an extension would require a change to the treaty, and would have to come before 1 July.
Most European deadlines are flexible, but rewriting a treaty would require plenty of time, and also goodwill from across the EU. And there are lots of countries where patience with the UK is running thin.
So if the government does decide to give itself more time to “Get Brexit Done”, it will have to make that call within the first half of the year.
For some of Mr Johnson’s supporters, that will surely feel like a backtrack. Whatever happens in the coming year, getting Brexit done won’t be easy.
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