British Prime Minister Theresa May submitted a letter triggering "Article 50" today, kicking off the formal process for the United Kingdom to leave the European Union.
Divorces can be messy - and drawn out. Anger, regret and coldhearted calculations can lead the two sides to an agreement that neither side wanted or expected. There is no precedent or guide for any country to leave the EU, but these four factors will play a key role in the coming months:
1) The real negotiations begin . . . well, not quite yet.
In the nine months since the June 2016 Brexit referendum, there has been a calm before the storm. Ironically, this period has seen Britain, often portrayed as the awkward member, often at odds with the EU, as a quiet and constructive partner on day-to-day issues in Brussels.
Although there will be a quick formal E.U. acknowledgement of the letter triggering Article 50, we will have to wait after that until April 29, when a special European Council meeting of all the heads of government of the other 27 member states will discuss the guidelines for the process.
But two EU founding members, and arguably the two most important countries in the negotiations, face elections in the coming months. In France, the election of Marine Le Pen as president would change the entire calculus of negotiations, but this outcome looks unlikely at the moment. And we won't know who will govern Germany until after the September elections - so the real negotiations won't get going until fall 2017.
There is no manual for how a country can exit the EU The now infamous Article 50 of the EU treaty offers little by way of a practical guide. There will need to be negotiations on how to conduct the negotiations.
There are thorny questions to resolve at, or near, the beginning. Money is a big issue - the U.K. has been a major financial contributor to EU Indeed, the anti-EU campaign bus last June produced a defining image of the referendum campaign, depicting 350 million pounds ($434 million) a week of UK taxpayers' money heading to EU.
Who will cover the pensions of UK officials who have worked for the EU? Should the U.K. fulfill its commitments under the current EU budgetary arrangements? Is the UK entitled to walk away with some of the EU's assets? Agreeing to a final Brexit price tag will be challenging - and probably involve tactical retreats and concessions on both sides.
A second big issue is the rights and status of EU nationals in the UK and UK nationals in the other 27 member states. Should these citizens be accorded the same rights and access to welfare benefits, among other privileges? Both sides are under pressure not to use the rights of these citizens as bargaining chips in the negotiations.
A third hot topic is how much of the negotiations will focus on the UK's future relationship with the EU Article 50 simply refers to the agreement on exit "taking account of the framework for future relationship with the Union." Given the timescale involved, especially the need for any agreement to be ratified by the member states, it seems unlikely much more than an outline of a future framework deal will be agreed to by the time the Article 50 negotiations conclude in fall 2018.
3) We can expect turf wars and multilevel chess.
Political scientists like to refer to two-level games, but the process of Brexit will be more akin to a game of multilevel chess.
The past few days have reminded May of the challenges she faces holding together another union: the United Kingdom. Scotland's Parliament has backed First Minister Nicola Sturgeon's call for a new independence referendum.
The Northern Ireland Assembly elections have led to a stalemate over the formation of a new administration in Belfast. Brexit only complicates the Northern Ireland imbroglio, as the Republic of Ireland is an EU member and shares a land border with Northern Ireland.
On the EU side, demands will likely be just as complex. The main institutions will want to exert as much influence as possible over the negotiations. The Commission and the Council will be central to any deal, but the consent of the European Parliament will be needed for any agreement to come into force.
The history of negotiations involving the EU is full of examples of states seeking to use the need to reach a deal as a means of extracting concessions. Although Article 50 only formally requires the support of a qualified majority of EU member states, on such a salient a step there will be a strong desire to have unanimity.
Central to the motivation on the EU side is the fear that Brexit might set a precedent. As Catherine de Vries argues in a forthcoming contribution to the JCMS Annual Review of the European Union, there is a desire among some on the EU side to "benchmark" exit from the EU as a distinctly unfavourable alternative for other member states.
4) Brexit won't be quite like integration into the EU, in reverse.
Leaving isn't as simple as joining, but the EU accession process can offer some pointers. Studies of EU enlargement show that negotiations go to the wire - and agreement often is reached when states strike deals to grant temporary exclusions on sensitive issues, even after the summits were supposed to end. So this is what we might expect to see as the negotiations are due to conclude in fall 2018.
Disentangling a marriage that lasted four decades will be no easy task. The tougher and more complicated negotiations, however, concern not the act of leaving the EU, but the UK's future trading relationship with the EU Given that striking trade deals and getting them ratified is challenging at the best of times, especially given the EU's complexity, a transitional package looks highly likely, with negotiations on a more comprehensive trade deal stretching out into the distance.
In the coming months and years, we should expect many meetings, threats, broken promises, reluctant concessions, the occasional slice of statesmanship and awkward partners on both sides.
• Tim Haughton is associate professor in European politics and head of the Department of Political Science and International Studies at the University of Birmingham.