LONDON — So now we know. U.K. Prime Minister Theresa May will trigger Article 50 in the final week of March, setting in train Britain’s two-year, slow-motion exit from the European Union.

For all the anticipation, the occasion threatens to be as dull as it is momentous. Formally, all that is required to begin the process of disentangling 40 years of political and economic cooperation is a letter. An email would be enough.

But then what?

Privately, even the most senior U.K. government officials closely involved in preparing for the divorce admit much of what happens after Britain formally begins negotiations — “B-Day” as it is being referred to in Westminster — is unclear. “We’re in uncharted territory, to be honest,” is the assessment of one senior Tory aide with as much knowledge of the process as anyone in government.

One point is clear in the 259 words that make up Article 50: Two years after its activation, the U.K. will no longer be a member of the EU, whether or not a deal has been struck on the terms of the divorce.

For a deal to be concluded within this deadline, both sides need to have reached an agreement in just 18 months, October 2018, leaving enough time for the U.K. and European parliaments to sign off on the divorce terms.

Only after the mandate is agreed and the two sides have negotiated over how to negotiate will the two sides actually get down to the divorce package.

Officials on both sides privately admit this time frame is optimistic. In reality, diplomatic sources told POLITICO, EU leaders will not finalize their own negotiating “mandate,” the framework which the Commission has to stick during the talks, until around three months after Article 50 is triggered.

For Brexit, time is short but the road is long. Here’s POLITICO’s route map for the months ahead, including the crucial 12-week diplomatic opening at the beginning of the negotiations when the EU will agree how it will approach the road ahead.

Triggering

The Article 50 notification itself will come in the form of a physical letter to the European Council with “some kind of handover” expected, a U.K. official familiar with the planning said. However, it is yet to be decided, even at this late stage, who will deliver it. Sending the notification via email has not yet been ruled out.

Britain’s Ambassador to the EU Tim Barrow would be the obvious messenger. However, U.K. government sources insist that no decision has yet been made about who will hand it over or who will receive it.

A senior government source familiar with the thinking of David Davis, the secretary of state for exiting the European Union, expected that the letter would be in May’s name, positive in tone (crucial to getting off to a good start, No. 10 believes), and would reiterate the U.K.’s position that both the terms of exit and the new relationship should be settled, in parallel, within two years.

There is unlikely to be any new information about the U.K.’s goals, as Downing Street considers the recent Brexit white paper to be its final word on the subject until negotiations actually begin.

Tusk’s response

European Council President Donald Tusk said in Brussels last week that he would need just 48 hours to respond to the U.K. with “draft guidelines” for the negotiation. The U.K. government does not expect this to be very detailed, Davis indicated on the BBC's Andrew Marr show over the weekend.

The Tusk response may, however, formally set the date for an EU27 summit that will “finalize” the negotiating guidelines — the principles that EU member countries want upheld in the final deal.

Extraordinary meeting of the EU27

Tusk has said this will take place “probably in April.” Had May decided to trigger this week, then EU27 leaders would likely have met on April 6, German Chancellor Angela Merkel said at last week's European summit.

May’s decision to wait until the end of the month threatens to push the meeting back to later in April, or even May. This risks the meeting clashing with the French elections (first round April 23, second round May 7). The French are seeking to avoid such a scenario and in the event of a late triggering will likely push for the EU27 summit to be pushed back to later in May, diplomatic sources said.

This summit will be critical. The overarching principles of the EU’s negotiating mandate will be set here.

Davis explained the process from Britain’s perspective on the BBC on Sunday: “The Council has to decide a guideline — they tell the Commission how to carry out the negotiation. That will require a meeting of the Council, which will probably take a month [after Article 50 is triggered]. They hand that guideline back to [European Commission President] Jean-Claude Juncker. Then we meet and then we start.”

The mandate

But it’s not quite as simple as Davis suggests.

Davis is right that after the extraordinary meeting of EU leaders in April or May, draft guidelines of the negotiating mandate will be sent to the Commission, where heavy lifting really begins.

A detailed plan for how the Council’s negotiating aims can be achieved will then be drawn up in the form of "Council directives," which will then be sent back to EU leaders for a final agreement. It isn't until this stage that the EU’s chief negotiator Michel Barnier will have a "mandate" within which he will work in the negotiations with Britain.

This period — between April/May when the directive is sent to the Council and June/July when the mandate is set — is crucial.

If the U.K. wishes to influence the EU’s opening position, it needs to get in early, ensuring allies such as Ireland, Poland and Hungary fight to ensure the “mandate” is not so hard-line as to make a deal politically impossible for the U.K.

Officially the Council’s position is agreed by a qualified majority vote, but it is customary for EU leaders to look for consensus on such important decisions.

Once the mandate is set, it’s set. With barely 18 months to agree on a deal, it is in neither side’s interest to reopen the negotiating positions once they have been agreed.

Discussions will take place in “mostly Brussels, but not just Brussels” — David Davis

“It’s quite a substantial amount of time,” a senior member country official familiar with the process said when asked about the opening three-four month window to agree on the mandate. “I think they’ll work very hard to get the mandate right with a view to avoiding amendments once the negotiation is underway. Because if you allow that to happen once, every time some issue comes they’ll try and amend the mandate.”

Back to the Council

The final rubber stamp on the EU’s negotiating mandate will be given by EU member countries. This could come as early as May but may stretch on until July.

This may be dealt with at the level of the General Affairs Council — which is made of the Europe ministers of member countries — or could be escalated to leader level at another extraordinary meeting, depending on how easy it proves to reach an agreement.

After it's agreed, the GAC will continue to be the accountable body for the negotiating mandate — i.e., the only body with the power to change Barnier's red lines. “It’s quite exciting for them,” said one U.K. government official. “Usually they only have quite dry material to look after, but now they’re working on Brexit.”

Negotiations about the negotiations begin

On Sunday, Davis explained how he saw the talks progressing: “The first meeting, bluntly, will be about how we do this — how many meetings, who’s going to meet, who’s going to come, specialists, generalists, whatever.”

Discussions will take place in “mostly Brussels, but not just Brussels,” he said, suggesting that important decisions could be taken in national capitals.

Davis expects that whenever the two sides cannot agree on something — whether it's over money, citizens' rights or anything else — the decision will be escalated to EU leader level for a political solution.

“The formal negotiation will be between the U.K. government and the Commission on behalf of the 27 member states, on behalf of the Council," Davis explained. "At certain points along the way, there may well come points of contention — let’s imagine we don’t agree on something. I suspect at the end of the day it will be the Council [that intervenes].”

Finally, the negotiations

Only after the mandate is agreed and the two sides have negotiated over how to negotiate will the two sides actually get down to the divorce package.

At that point — "sometime in June or July," one senior diplomat of an EU27 member country predicted on condition of anonymity — the Dutch and French elections will have been and gone but Merkel’s reelection bid will just be gearing up.

Davis told the BBC that agreeing on a deal on citizens’ rights would be “the first thing we will deal with” when the talks get under way.

After that, the negotiations are expected to turn to money and the £53 billion Brexit divorce bill that some in Brussels have suggested Britain must pay. The Commission has argued that only once this exit fee has been agreed can the talks turn to Britain’s future trading relationship with the EU, which Michel Barnier expects to take up much of 2018.

So, three to four months to agree on a mandate, six months to deal with the exit package — including “the check” — and six months to agree on a comprehensive free-trade deal. It took Greenland three years to negotiate its exit in the 1980s — and all they had to discuss was fish.