Forty-four years ago, the United Kingdom joined the European Union. Nine months ago, in a stunning outcome, a slim majority of U.K. voters opted to leave in a national referendum. On Wednesday, Brexit formally began when Britain’s envoy to the EU hand-delivered a letter from Prime Minister Theresa May to European Council President Donald Tusk informing him of London’s intention to leave the union and commencing a two-year negotiating process over the terms of Britain’s withdrawal and their future relationship. Both Europe and the U.K. have entered uncharted waters, and the talks and the outcomes are likely to be bruising for both.

Britain seeks to regain sovereignty over core national concerns, in particular to reclaim control over immigration and exempting itself from the jurisdiction of the European Court of Justice, while retaining access to the European market. As May has argued, “While we are leaving the institutions of the European Union, we are not leaving Europe.” European governments, while hoping to maintain good relations with Britain, do not want to signal a readiness to accept “a la carte” membership — permitting members to pick and choose their obligations and benefits. Too favorable a deal could encourage other governments to follow Britain. The EU position, as explained by Tusk, is simple: “To minimize the costs for the EU citizens, businesses and member states.” That means that at a minimum, Britain cannot get a better deal in the future than that which it enjoys today.

While negotiations over the terms of Britain’s withdrawal and subsequent relationship will be extremely difficult, that challenge is magnified by London’s desire to work out details of both simultaneously. EU and European officials counter that only after withdrawal has been agreed can they then consider their future relationship. In addition, elections that will be held this year in France and Germany will slow the negotiating process as those two governments will be central to shaping the European position in the talks. Since the EU treaty stipulates that the process must be finished within two years — a period that includes ratification of the deal by EU member states — time constraints will have a powerful impact on the outcome.

Brexit is also likely to have an equal, if not greater, impact on Britain’s internal politics. While the overall vote was close — 52 percent to 48 percent in favor of leaving the EU — the four U.K. countries had profoundly different preferences. England and Wales wanted out. In contrast, 62 percent of Scots voted to remain, while nearly 56 percent of Northern Irish wanted to stay. As a result, Scottish parliamentarians voted earlier this week for a second referendum on their independence. In June 2014, Scotland voted 55 percent to 45 percent to stay in the U.K.; independence-minded activists argue that Brexit constitutes a material change in conditions that justifies a second ballot. Scotland’s first minister, Nicola Sturgeon, has pledged to hold another referendum on Scottish independence in late 2018 or early 2019. Northern Irish advocates of unification with Ireland make a similar case, insisting that Brexit threatens the basic premises of the Good Friday accord that brought peace to that troubled province and would deepen the division of their island.

If the particulars of Brexit remain unclear, the broader implications of Britain’s departure are not. There is the symbolic importance of being the first country to leave the EU, and the appurtenant realization that greater union is neither eternal nor inevitable. Postwar European politics has been premised on the idea of expanding solidarity and greater integration; Brexit suggests that neither is true. That may not be a bad thing. Too often, the political idea of Europe has been taken for granted, a neglect that is evident in complaints about a “democratic deficit” in the EU. Now, European politicians will have to work harder to validate beliefs in and win over supporters of European ideals and structures.

A second implication concerns Britain’s foreign policy making. Britain is the world’s fifth-largest economy (as long as Scotland is included) and the EU is its largest trade partner. Not only does the end of EU membership change the terms of that trading relationship, but it also means that Britain will now be responsible for negotiating its own trade agreements — and they all must be updated or created. That is an extraordinary burden for an office that has atrophied since joining the EU.

London has retained more autonomy in foreign and security policy, but it will have to reorient in the aftermath of Brexit. Some see this as a benefit for Britain, as it will have more flexibility and freedom after Brexit, but others fear that London may lose weight and influence with divorce from the 26 other nations of the EU.

Whatever the outcome, one thing is certain: The process will not be easy or painless. EU governments have no interest in letting other members even entertain the thought that exit is worth the trouble. That is not vindictiveness, but self-interest. Britain will now see if its calculations are as clear-headed.

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